The Forgotten Skills of Self-Sufficiency used by the Mormon Pioneers (I wrote about it in a previous post.).
I've only glanced through it, but I zeroed in the pioneer yeast section. The author, Caleb Warnock, talks about starting your own yeast or buying a starter from reputable dealers. Warnock says in his book that he got a starter from a friend who got her's from http://ldshealth.ning.com/. I looked up the site and it looks like a jackpot of wholistic eating.
For those of you more intrepid, you can make your own starter. Here's a starter recipe: http://pioneerfoodie.blogspot.com/2009/02/so-then-yall-may-have-seen-my-recent.html.
Here's another site about sourdough starters which also lists commercial places to purchase a starter: http://www.sourdoughhome.com/startingastarter.html. I recognized one of the sites as a manufacturer of gluten-free products. While the starter isn't GF, they do have several GF products: http://www.kingarthurflour.com.
It looks like most starters use gluten-containing flours, so it won't work for those of us who need to be GF. I did find a site that talks about GF starters and fermented drinks like kefir, which is supposed to have pro-biotic properties: http://www.thefreshloaf.com/node/12481/yeast-free-gluten-free-dairy-free-sourdough-starter.
There is a gluten-free starter recipe listed at this site: http://gluten-dairy-sugarfree.com/?p=884, and the comments under that post have additional helpful information.
Another site that has fantastic information on yeast in general and had a link to another GF starter recipe is: http://blog.julesglutenfree.com/2011/04/yeast-answering-the-questions-that-also-arise/. The site she links to is: http://simplysugarandglutenfree.com/sourdough-starter/.
If you have any other sites to add, please post them in the comments section. I'm always looking for good gluten-free info and healthier eating options. I've only scratched the surface of info out there, but the kids are awake and I'm in desperate need of a cleaning or I may become a starter of my own.