My farrier sells QH and if they're backed then they are Western trained. What level are you looking for? Something that has just started reining or a proper show horse?
I suppose it depends where in the country you are, the QH and Appy society would be a good palce to start they do classifieds as far as I am aware.
There is also a magazine called Free Rein which is for all the USA breeds, and includes lots about quarter horses and western riding.
The Western Equestrian Society would also be a good place to start, they have lots of clubs and branches throughout the country who hold regular clinics, shows, etc.
www.aqha.uk.com www.thewesternshop.com
Sales lists on both.
But beware, a fully trained QH that is suitable for someone novicey or with a disability is very sought after and thus v expensive.