I use Model Mayhem a lot.  It works well for the most part although I get a number of no-shows and last minute cancellations.  Generally I plan to do three shoots on the same day and expect at least one no-show.

For normal commercial work I pay $75 for a two-hour shoot, sometimes less for models with no or limited experience.  I do some fine art nudes that I typically pay $200-$300 for two hours depending on experience.

I also try to get people I meet in public and offer TF.  That works sometimes but cancellation/no-show percentage is probably 75%.

Some people swear by Craig's List but I've had no luck that way.