funny you should mention an assistance dog. I just am studing them cuz i want to be a trainer for the programs. here are a couple of places. Most use rescue dogs from local shelters.
Canine companions for independence
po box 446
Santa Rosa, ca 95402
8005722275

The Delta socitey
service dog resourse center
289 permenter rd east
Renton, Washington 98055

Thry are training dogs for all kinds of help partners and all fall under the disabilties act so the are admitted everywhere a guide dog is. I just have been amazed the stories I just read.