Shelters often have adult dogs with some training that ended up there through no fault of their own. TELL them you are looking for an ESA before your first visit and they'll curate suitable dogs for you. You may also find a trained adult dog through Petfinder, although you may have to be a bit more careful there...those dogs are more likely to be listed for a reason.

Breeders also sometimes have adult dogs available.

One last recommendation: You're living in a dorm. Get a small dog that's not prone to barking. I wouldn't go about 20, maybe 25 pounds, and avoid dachshunds - they're amazing dogs, but I have yet to meet one that didn't want to sound off constantly! You don't want a dog that makes you unpopular with the neighbors and a natural non-barker means one less training issue (which can be particularly tricky) to deal with.