Hi and welcome!
While I have a couple of labs, they are not therapy trained. They are my constant companions, though, when I'm not working. My first suggestion would be to check out some of the threads on this board that go over what sorts of things to look for in a breeder. Then start looking for lab breeders in your area or close enough that you would be willing to drive to pick up your new pup, assuming you want to start with a puppy. While there are lots of dogs in rescues or shelters that may be trained for different types of therapy work, it sounds like you want to find a dog that you can be fairly certain that its temperament is that of a well bred labrador retriever. There are many people who breed labs but not all necessarily take the care needed to make certain the dogs they breed have all the health clearances for orthopedic, cardiac, eye, or other genetic issues common to labs. There are breeders who advertise online who appear to talk a good game and have a good site, but they may be brokers or puppy mill breeders who breed in large numbers without taking the care to make sure the dogs are worth being bred to begin with. Reputable breeders will be aiming to breed dogs that are healthy and have the lovely temperament that labs are known for.
You could also look to see if there is a labrador retriever club in your area where you could get to know some other lab loving folks, breeders and trainers. Once you get a certain number of posts, 5-10, people on here who do know of breeders around the country might be able to send you private messages to point you in a good direction. We are not supposed to name or recommend breeders or kennels on the open forum.
Asking About Breeders or Where to Get a Puppy
How to spot good Labrador Breeders
Good luck- let us know how the search goes!