I happened to get mine via eBay, from Australia, a very nice 120mm one with stand and box. I think I paid only about $35 due to being the only bidder, but the shipping kinda whacked me.

I also bought one of these, more for halloween, but it puts a bit of special FX "magic" into the crystal ball..
http://www.lumisource.com/product_detail......t_id=170
The Lumisource Electra Plasma Ball - it's opaque, as opposed to translucent, and has no center rod from the which the plasma emits, instead, the whole inner surface of the glass ball is coated with a phosphorus compound. I got the multicolor version, which looks more neutral.

I use a 4 inch dark green crystal ball. For a stand I have a silver antique bowl that I turn upside down. The base is concave and the ball sits right in the recess.