There are some resources online about how to go about being a professional narrator, like this one from Audio Publishers Association: http://www.audiopub.org/resources-narrator.asp

Here is information from NLS about employment opportunities with their studios: http://www.loc.gov/nls/studio/index.html

LearningAlly has volunteers record books, so you could contact them for volunteer opportunities: https://www.learningally.org/volunteers/

Some, but not all, network NLS libraries have volunteers record books and newspapers and magazines. You can look here: http://www.loc.gov/nls/find.html to find a NLS network library near you and contact them about volunteer possibilities.