There is still a ton of P3 stuff available, and a lot of P4 stuff will work in P3. Your request is kinda broad to reply helpfully. Whatcha lookin for?

That said, the next three largest Poser related free sites with downloads are probably 3D Comic Collective, 3D Commune and perhaps Lannie's 3D ModelWorld.

Also bear in mind that any Head morph targets for P4 figures will work on the corresponding P3 figure.