1) Pet Stores: They are everywhere. Some have "fancy" rats and others have "feeder" rats. Really, feeder rats are usually just black hoodeds or abinos (called Pink Eyed White, PEW, in the fancy) that aren't as popular among the pet buyers.
2) Animal Shelters: Many animal shelters have a small animal room and there are rescue organizations dedicated to small animals. Adopting an older rat is a rewarding experience and you don't have to deal with the hyperactivity of a young rat. This is also a good option if you are looking for a single rat since shelters may have "problem" animals, which with rats may mean they show aggression towards other rats and need to be housed alone. These same rats, however, are absolutely wonderful with their humans.
3) Breeders*: This can be a tricky one. On one had you can get some amazingly friendly rats from breeders that come with pedigrees and knowledge of the prospective health of the rat. However, I would make sure you get to know the breeder, see how they raise the pups, and meet the parents of the babies. Online photos can only tell you how pretty they are, but temperament is best judged in person. Breeders usually require babies to go home in pair but typically offer to take the babies back AT ANY TIME if a situation arises that they cannot be kept for their entire life.
In a pet shop or animal shelter you typically go, hold them animals you are looking at and judge the best fit for you. This should be no different with a rat breeder. I have gotten some of the most amazing and beautiful animals from breeders, the kind that give you kisses, but I have gotten amazing animals from shelters and pet stores, too.
Now, you probably want something cuter or more exotic than a black hooded or PEW, or even a deep brown agouti. You have spent some time looking at rat photos online and now have your heart set on bringing home a dumbo rex chocolate dalmation. You probably won't find those at the pet store or animal shelter, or if you do it is a rare occurrence. That is where a breeder will come in handy. Ratster (http://ratster.com/) has a state by state listing of breeders and some animal shelters that specialize in rodent rescue. This is a good place to start searching for breeders that might have what you are looking for in your new pet. Just make sure to call or email to double check they are still breeding, are still breeding what you are looking for, and to get on a wait list.
That is probably the biggest problem with hobby breeders. They work towards breeding a specific type and are usually going to be scheduling litters around a local rat club's show schedule. So, there will be a wait list. You can be waiting 6 months to a year for your perfect rat. There are so many factors that a hobby breeder needs to balance when trying to adopt out babies and make sure they aren't over breeding their wait list. Sometimes litter sizes are smaller than anticipated, sometimes the litter just completely fails. Rat breeding is not exact and can be extremely heart breaking. Hobby breeders are not doing it for the money, but for the love of rats. So each litter is precious.
No matter where you choose to adopt your first rat it will be a wonderful experience. Treat the rat the same was as you want it to treat you and the two of you will have a wonderful friendship. Making sure that you have at least 2 rats per cage gives you twice the amount of love and affection as well as insuring that your little friends are never bored or lonely when you cannot be with them. Many people start with one rat, get a second to be a friend, and then end up with a pack of four so that they are never without a rat.
The downfall of choosing rats for pets is the short life span. They reward you with a life full of joy, but sadly they are a 1-3 year pet. With an average life span of 2 years they present a shorter commitment than a dog or cat, but just as much (if not more) love and enjoyment of life with their humans.
*I am a breeder but do not want this blog to become about my rattery, will not post the URL to the blog, but you can find the URL in my profile.