There are lots and lots of buildings you can get for free. Remember to look at things in .3ds format, which you can import into Poser. There seem to be far more free buildings available in .3ds than things that are Poser ready. Add to the above sites 3D Cafe, mapps's site, and Baumgarten. If you need simple things like walls, scaling primatives is an easy way to do it. I just got Blender which is a free 3d modeling program, and pretty good with good documentation. Poser has the tools within it to be a kinda simple modeling tool if you don't get too complicated. With enough tweaking you could get the primatives to make what you wanted. But that time you spend tweaking could instead be devoted to actually learning how to model 3d.