There are numerous ways to acquire a pet rat. And certainly one of the best ways to do that is going through a local rescue organization, going through a sanctuary, or working with people that take in unwanted or rats that result from unexpected litters.
You can often find rat rescue organizations online. You can search for them. You can check with any type of local veterinarian that sees and treats rats.
You can also check with people that own rats and find out where they got them from. But I am a strong advocate of recommending that you attempt to go through a rat rescue organization as a means to acquire them.
Most of the time people that do rat rescue have a number of different type of rats, a lot of different varieties, a lot of different color patterns. And a lot of the times they have young rats that are going to be ideal to take home to socialize and train. So there aren't any concerns in feeling like because you rescue, you necessarily have to take an animal that may have behavior problems or may have health problems. If anything, with rats, it's often the opposite of that.
So If you choose to acquire a rat, and you don't need a very specific breed for some reason, and you're willing to just open your home to having rats, I would strongly recommend that you consider going through a rescue organization, seeking out and finding somebody who takes in and fosters rats, people that work with taking in rats through hording situations. And open your home to rats form these types of environments that otherwise aren't being sold and otherwise would have no place to go.
So consider adoption. Consider working through a rat rescue organization. Seek out and find people that work with the re-homing of pet rats. And open your doors to adoption.