They're selling fairly large prisms on e-bay for like $30... They're close to the dimensions you asked for too. Here's a link:

http://cgi.ebay.com/Vintage-optical-...item439b693c6a
I bought a couple of these - bad news is that they are 90/45/45 angles, not equilateral - they're surplus from submarine periscope manufacture!
Good news is that the glass quality is very high, and it's perfectly possible to split white light into a spectrum with them, just takes a bit of fiddling with the angle of incidence. Mine also came in nice purple velvet bags...