The main forum for France is
Youppie.net but, of course, it is all in French. Nevertheless, it is a great forum with tons of useful info.
However, I would not recommend doing anything in France. The country passed a "Nordic prostitution law" several years ago. It criminalizes the client and not the provider. Unfortunately, unlike most of Canada, the law is actually enforced. Unless you are really savvy and experienced, you probably shouldn't pay for sex in France. You risk getting arrested.
Instead, consider a quick trip to Germany--that's what the French guys do. There are a few FKK clubs just over the border that are very convenient for guys living in or visiting France. The biggest and best one is in Saarbucken:
FKK Paradise. High speed TGV trains run from Paris to Frankfurt several times a day and some of them stop in Saarbrucken. You can get there in about 2-3 hours from Paris. That's a short enough time to make a day trip. Once you're in Germany, you have no worries about breaking any laws about prostitution.
I don't think you should risk paying for sex in France. Either go to Germany or save your money for your next trip to Montreal. You can have a wonderful time in France. Enjoy the food, the culture and ogling the beautiful women, but stay away from commercial sex.