I've noticed in my area that Craigslist have definitely gone downhill with regard to quality bikes. The only decent ones seem to be either unpopular sized-frames or newer bikes and many are overpriced in my opinion.

Having said all of that, I'd be looking for a mixte frame for a conversion. And if Craigslist is not yielding results, check bike co-ops as suggested and also Goodwill and similar. Hopefully a good candidate will appear.