Eugene,
First of all, your time of vacation is pretty ideal. The crowds are moderately sized at that time of May. You won't get to walk right on to each ride, but you won't have 2-hour waits for each attraction either. I think May is the second best time of the year to be there, after the September/October time frame.
As for your root-beer question, no, I don't know of a list anywhere that shows all dining locales that serve root beer. However my experience is as follows;
- Most table service restaurants do offer it
- Most counter service restaurants do not
- Most resort food courts do offer it
- The brand of choice (as far as I have experienced it) is Barq's
If you need to make absolutely sure that a place has root beer, you can view the menu for that restaurant at it's page on the Disney World website. If one is not available, you can visit AllEarsnet.com, go to their dining section, and view their menus.
I hope that this helps?
I am glad to field any follow-up questions. Just visit my bio page to ask!
Have a magical vacation!