I agree with Jim, it is so quick & easy to make a very good true ground glass that I don't see the point in a lot of other (and what seem to me to be inferior) methods that folks try. A small bottle of 600-grit silicon carbide from a rock-hound (lapidary) or hobby shop will last you forever and make many, many sheets of ground glass.

My method is basically the same as described. I've found that hobby shops carry thin glass (thinner than window glass) pre-packaged for use in framing. If you ask nicely they'll usually cut it for you for free, or for a small charge - although that's really not hard to do you yourself either. I take 2 pieces of 4x5 glass, cover one side of each with package tape as smoothly as possible not to make bumps or lumps in the surface, put the grit between with a little water to make a paste and rub them together in a semi-random orbital fashion. I put the bottom one on a piece of flat masonite, or any flat surface and swap them out from time to time so that they both get ground evenly.
Wash them off occassionally to ck your progress. Takes 15-20 mins max. Wash 'em off, take off the tape, and you've got 2 good pieces of GG.

And yes, I do plan to buy a new one from Dave soon, too cheap (for that quality) to pass up!