You can find all sorts of freebies here and at other sites. Start by clicking the Free Stuff link at the top of this page. And visit the 3D Comic Collective for all sorts of downloadable characters.

You can make your own characters and props, but at the very least you'll need a paint program (for making textures) and some familiarity with Poser. New characters can be derived from the existing figures by means of morphs, which you can create using Poser's magnet tools. If you're really ambitious, you can create new geometry in a 3D modelling program, then convert it to a Poser-ready figure. Renderosity has many tutorials on the subject.