The big three in the US tend to be (in no particular order): Guide Dogs for the Blind, Guiding Eyes for the Blind, and the Seeing Eye. There are other schools which aren't bad, just not as well established or known.

There are also fly-by-night regional operations that you should be suspicious of that often make you pay substantial amounts for a dog. Whatever the school is, you should always ensure it is certified by the International Guide Dog Federation (IGDF).

When I went thru my mobility training I asked this and they recommended The Seeing Eye, our of Morristown NJ. It’s one of the oldest schools in the nation that can trace its history all the way back to World War 2. They offer several programs to accommodate just about every candidate. I’ve never actually gone thru training at any school but this is my choice once Covid calms down.