Assistance animals are typically trained from the time they are young pups rather than training a person's existing pet. I know a gentleman and his wife who train Seeing Eye dogs. I was amazed at all the training, and even more amazed by the fact that even after all the classes, some dogs just can't pass the exams to qualify them as assistance animals. (I think they had a hard time filling in the little bubbles on the score sheet since they can't hold a #2 pencil <img src="i/expressions/face-icon-small-smile.gif" border="0"> ) Seriously though, the graduation rate in the assistance dog training industry is only 20-30%. You'd be hard pressed to find a place that will train a dog that has been rescued, or one that isn't from the company's own breeding program.
Here is a link to a list of agencies in the US that do assistance animal training. <a target=_blank class=ftalternatingbarlinklarge href="http://www.inch.com/~dogs/service.html
">http://www.inch.com/~dogs/service.html
</a>
Golden retrievers and labs (or a mix of the two) are most often the breeds that are trained to be assistance/support dogs. In CA the fee to obtain an assistance dog is around $2500, and the waiting list is anywhere from 6-12 months.