The simple answer to your question is it depends what you're looking for. If you want glamour models then both OMP and ModelMayhem are great. OMP has better search features as they allow models to classify what they will and won't do. MM seems to be a bit more fun as a community.
Another place to look would be your local life modelling community. I've found, in London at least, that life models tend to be a bit more artistically minded. And their fees tend to be lower too. But don't ask a life model to, "do some glamour stuff," as one of mine was - unless you want a very confused model...
Either way I’d counsel being sure in your own mind what you want to achieve (so you can describe this to the model), involve them as much as you can (e.g. through polaroids), and treat them as valued people not as objects (i.e. respect them). That way you’ll get much more out of your joint work and won’t join the list of GWC horror stories that they’ll happily tell photogs they trust.