It doesn't matter whether they are for free or for purchase as long as they are compatible.
The best commercial packages of themes for the iWork apps, in my experience, are those from Jumsoft. They produce themes for both Pages and Keynote, as well as animation and art packs.
For free templates, iWorkCommunity has a ton of Pages templates, and some decent Numbers templates as well for basic use. KeynoteUser also produces commercial themes, in addition to having some freebies, and an excellent blog and collection of links for other sources of high quality stuff.