I've always had crappy to no results using a quartz crystal ball or plain silver mirror. Dunno why... *shrug*
Obsidian balls aren't that expensive. A decent size one will run you between $30-60 bucks on average, but that's for a good five-inch diameter ball. Look around at rock and gem shows. A lot of dealers, if they like you, will give you deep discounts. I grew up w/a lot of them here (hitting rock shows has been a hobby since childhood) and paying full retail is unknown to me. (IMG:
style_emoticons/default/wink.gif) A lot of carved obsidian, onyx, and miscellaneous other stone comes out of Mexico. Check out some of the local shops next time you're in Tijuana or Cancun.
Never tried a black mirror, but I'm tempted to get a picture frame and a couple cans of black spray paint next time I'm at Home Depot and see what I come up with.