We make those stone retreats ourselves. Lots of masonary tools and sweat. We have a website with examples if you would be interested in checking them out. They are made from volcanic rock. These are great for your display tanks. We hand-carve each one and make small, medium, and large. Our website has the dimensions and we do custom orders. http://www.geocities.com/tarantulakeepers/
We'll answer anymore questions via e-mail.
Yeah, sorry far too large pic. I thought I had already reduced the image but apparently not. Check these out, we have worked out arboreal designs using real tree branches that are coated to prevent mold or rot. They are non-toxic and we use this one in our P. irminia enclosure: