Cheap, available, and the dairy industry does my QA for me. :) Milk for yogurt is pasteurized before it's inoculated with cultures, so I'm reliably getting just L. acidophilus, L. bulgaricus and S. thermophilus (verified by gram stain and visual identification).

There's no real "official" guide to biohacking, but openwetware.org is a great place to start, as is the DIYbio mailing list, diybio@googlegroups.com.