What you are describing is what is commonly referred to as a "therapy dog". More often than not, your dog will need to prove itself capable of tolerating high stress situations through an official evaluation. As well, your dog will most likely be required to earn its Canine Good Citizen (CGC) which requires that your dog be well behaved and socialized in itself (your dog does NOT have to be purebred or registered with the AKC to earn its CGC).

Here's a link:
http://www.tdi-dog.org/

You can also sign up to help foster guide dogs in training:
http://www.seeingeye.org/AboutUs.asp

http://www.eyedogfoundation.org/

You're helping a noble cause!