This is my birth stool:
http://www2.birthcenters.org/OnlineE...evesWood.shtml
I think with shipping (from PA to OR) it was $120. That is one of the less expensive ones I've seen. It's made from an Amish woodworker and he's totally wonderful.
Other options could be getting a portable toilet seat from a local medical supply place (some places even rent them)! I know it sounds odd, but really, what you need is something that resembles a toilet without putting pressure on your tailbone. When we sit on the toilet, we automatically relax and release our pelvic floor muscles. The toilet is ideal - but a portable toilet seat is something that's pretty easy to find. (My stool resembles a toilet seat, with a more open opening, so women sit on it like a toilet seat, but they do not sit on their tailbone)
Here is my ultimate dream stool:
http://www.acegraphics.com.au/product/equip/be001.html
I've also heard of midwives (and parents!) putting a toilet seat over a carved out 5 gallon plastic bucket.