In the general dog population I think research was that only about 1 in 100 dogs are able to handle the stresses of Service work. Programs have a better average with their breeding programs but that is because they have a lot more control over the parents and over how the puppies are raised.
WHile you can find good dogs at the Humane society, and several people have owner trained rescued dogs, you are really taking a chance on them because you will not know the history of the dog and there can be events in the dog's life that might affect how the dog reacts and works. This is why you see so many here recommending that you get the help of a professional trainer and a behaviorist to evaluate these dogs before you take them home, and why it is recommended that you use a professional trainer if you do not have a dog training background yourself.
I'm sorry your first dog can't handle public access, but glad that at least at home she is a help.