PolyCount, a low-res game modeling site, has more free downloadable characters than you could ever possibly use. The Half-Life section alone should fill all your needs; most of those are under 1000 polys.

I didn't mention the site before because all of the figures are compiled in proprietary game formats which Poser cannot read. But if you have MilkShape, you can decompile and import a Half-Life character, pose it as if it were a Poser figure, and then export the figure as a static .obj mesh.