Check Free, Casual, and Advanced for traditional RPs.  They may or may not be full, often they will have a FULL tag on them if they are full but not always.

You can check the Interest Check Threads for RPs that are either about to start or hopefully just recently started.

There is a 1x1 Section as well if you want to just do a RP with one person and a Nation RP section if you like to RP as an entire Nation.

There's also a handy dandy guide if you want.