Paha, it depends on what state you live in, but in most of them it's Not illegal.
But, I do have to warn you, you have to do immense amounts of research before getting either of these animals- they require a Lot of time and effort, and they can have many troubles growing up in captivity (they'll be sweet as babies, but once squirrels get near the age of adult-hood, they really want to be free and roam in the trees and will become nasty pets... and I'm not saying this because "these animals are WILD and need to stay in the WILD!!!" - most animals do quite well and are happy in captivity, I just can't recommend tree squirrels because most will turn out to be bad pets because of their wild desires, and don't bond to their humans as much even when amazing as babies and youngins. Although, if you're willing to go for SFS's (southern flying squirrels), they can make amazing pets and will bond to their human. I'm sorry, I'm getting even more off-topic, but there are things about both that don't make them great pets- for example red foxes are extremely destructive, and will mark everywhere with really bad smelling urine...)
And to answer your question, there are many breeders out there - if you let me know your state I could give you one closest by you that's recommendable, or if you message me I can try to find you a list of breeders.
I'm sorry I'm not answering it here, but I want to try to find you a good one that's close-by, and I know a lot of breeders...
(And also, if you'd like any information on them I can answer questions for you and give you info if you'd like.)
Sorry this isn't more helpful at the moment... good luck! =)