I have to chime in here.. I work with models all the time both in NYC and London and Paris.
ModelMayhem has a bigger range than PurplePort but try both.
However I would also just go down the model agency route, especially France/Italy/Spain...
If I'm shooting for my book our testing new kit or refining a technique I am shooting for me and don't want the model chiming in about how quickly will she get her prints and secondly.. if you add up the cost of your trip, clothes, MUA etc... then why run the risk of a no show...? I've had girls from the web who I had agreed to pay who cancel the night before... All that prep... for a no show.
Agencies actually offer you better girls, and rates that are cheaper than so called 'top' girls on the web charge per hr, plus there's no drama and normally a great work ethic...
I agree with Some Guy, try an agency or two first and remember there are a lot of 2nd and 3rd tier agencies out there with great models.
S