A rescue is an organisation or an individual that has taken in the rats with the aim of rehoming them. You may be required to undergo a home check before adopting rescue rats. I don't see this as a disadvantage because it shows that the rescuer is concerned about their wellbeing, although it can then restrict you to homing rats from rescues which cover your home area.
Rescue adults vary from the well socialised to the traumatised and aggressive. You may find yourself paying out a lot in vet fees and spending a lot of your time on an older rescue, but often this is the time when you bond most closely. It's a gamble but very rewarding to take on adult rescue rats.
Rescue kittens are generally those born to adults taken into rescue. Giving a home to young rats is less of a gamble than homing an older rat, especially if the rescue or foster home has spent time socialising the youngsters. You will most likely receive little or no information on their hereditary disease resistance or tendency to develop tumours.
There are many people who breed rats for many different reasons. What you need to be looking for is the 'reputable breeder'. This is complicated by the fact that there are as many definitions of a reputable breeder as there are people looking to find one, so what you really need to look for is someone whose ethics and viewpoint coincide with your own.
I would like to emphasise one point. Don't be reluctant to approach a breeder just because you are not intending to show your rats. Every breeder has kittens to home on occasion, and there is no reason to suppose that you wouldn't provide the kind of home they are looking for.
You will probably have to go on a waiting list for a breeder rat, especially if you are seeking a particular variety. If you ask to be added to the waiting list of several breeders, it is courteous to let them know if you have found your rats elsewhere, so please keep a record of who you have applied to.
One good reason for turning to a breeder is to find rats with good health and temperament. These are factors which can be bred for, so you are looking for someone who keeps good records of their lines and who uses these to influence their breeding decisions. These breeders will be keen to keep in touch with the new owners of their kittens. Ask about health and temperament.
You may also want to ask the breeder about their breeding practices and ethics:
Do they use a questionnaire to find out about prospective owners? You should be prepared to be given a questionnaire or be 'interviewed' for you suitability to take on your new pets. This shows that the breeder cares what happens to the lives they have brought into the world. Some breeders prefer to have an informal chat, others (like myself) prefer to take written details at first.
Will you be asked to sign a contract? Many breeders will ask you to sign a contract, with restrictions on the breeding and rehoming of their rats, and preferences for their welfare. Others will not, feeling that the contract is unenforceable, but will still have expectations as to the care of the rats.
Will you be asked to keep in touch? Regular updates help the breeder to monitor the health and wellbeing of their lines. You should expect to be asked to give regular updates.
Do they home their rats in same sex pairs or groups only? Rats are very social creatures. They need interaction with others of their own kind and preferably of a similar age. It is common for breeders to refuse to home single rats because of this.
Can you visit the rattery? This allows you to see the conditions in which the rats are kept at home, possibly meet the parents. It's a nice idea, but not always feasible given that some breeders are also working full time, have other commitments, may have rats susceptible to infection from visitors, or have had bad experiences in the past with bringing people into their home.
Do they do a home visit? Again this isn't always possible, though it would be an ideal.
Do they cull their litters? Does your breeder cull their litters down to make them a more manageable size for their does? Does this make a difference to your feeling comfortable with taking kittens from them and feeling comfortable with them as a person?
Do they cull or rehome their retirees? It's a question to ask, although I don't know anyone who admits to culling their ex-breeders.
Do they take back any rats that can no longer be kept by their new owners? This offer is another indication that the breeder is prepared to accept responsibility for the lives they have brought into the world.
Do they provide holiday rat sitting? As above, although this can be limited by space and other circumstances.
Buying from a pet shop tends to make the whole process very quick and easy, which, although it sounds good, isn't always the best thing. These are living creatures that you are taking on responsibility for, and you need to take time to consider this and to plan for their care.
Some pet shops do not guarantee the sex of the pets they sell. Many of the accidental litters offered for homing on rat forums and mailing lists are the result of pet shops mis-sexing their rats, or separating the male and female babies too late. The in joke is that you 'buy one, get twelve free'.
Pet shop rats are often separated from their mother too early, looking undersized and still in their baby fur when offered for sale.
Poor advice is often given on the size and type of cage required. To give the pets shops the benefit of the doubt, I'm guessing this is because they usually see tiny young rats and don't realise that they can grow into a relatively large adult.
The rats have no history given, often only an approximate age, and no information is fed back to the breeder. The only way that a breeder can select for good health and temperament is by gathering information through contact throughout the life of the pet. There is no facility for this from a pet shop rat.
The rats may originate from pet stock or a feeder/pet supplier farm, with overbred females and lack of selection for health. Farmed rats are not handled frequently enough during the crucial early period when young rats bond with humans.
The rats are sometimes not handled enough while in the shop, leaving the new owner with nervous or aggressive pets.
Because of these points, I believe that live animals should not be sold in shops, and would not support the shops by buying from them. Buying a rat from a pet shop helps the individual rat, but encourages the shop to restock, which in turn encourages the breeding of more stock. I'm not going to condemn anyone who does buy from pet shops, though, as I completely understand the urge to take the individual rat out of there.
It's quite common for older rats to be offered for rehoming, usually because their owner's circumstances have changed and they no longer have time for them. There are some very sweet rats to be found this way, but also some unhandled nightmares.