Besides placing casting calls on MM myself, I've answered casting calls placed by models, I've cold contacted models on here directly (verifying they've been online at least in the last month) and I've placed castings on craigslist.
Craigslist has probably been my best source for TF art. For models that have a it more of a commercial look, I've preferred cold contacting models here. I know some photographers use facebook.
When I started, I always paid, feeling I needed to have a reasonable portfolio to attract models for TF.