Nice to hear you want to do some portrait and figure work. I look forward to seeing the results. I only hope it will be LF or ULF.
I can't recommend any of the on-line sources for models. I have used them and do use them but think there are much better ways to find models. You might try placing an ad in the local college paper. This has had great results for me. They are a steady source of young models without the on-line BS.
Another source would be to contact local art schools in your area. They use figure models and someone who has posed for a drawing class is much better to work with using ULF cameras. They understand how to stay still and are not as easily bored.
Also look into any dance schools or companies in your area. Dancers have great bodies for figure work and are very comfortable with their bodies.
When I first got out of art school, I shot former classmates who I had taken drawing classes together with.
The best models Iíve found have been from showing my figure work locally. When Iíve done this it hasnít sold as well as in the big city, but I always get a couple of people who want to be in the work. They turn out to be among the best models to work with because they believe in the work.