I think you would have to find a specific program/school that has library assisting for a certificate. I have a friend that did children's librarian assisting through an online program at a college located at the other end of the state. I believe it was a two year program. You could go to your local library and ask them. If they don't have the answer, they will help you find it. They also might have shelving positions open that you could apply for to get your foot in the door.

Good luck!