If you need a piece cut other than square, you may have more luck at a glass shop. Look up Glass in the yellow pages, and you'll find a handful of glass shops in your nearby area.
I've seen them too... I haven't used them yet, but have called them about other tank projects that I haven't gotten around to yet and they had reasonable prices. They can also get glass tempered which is a nice plus
If anyone is looking for tempered glass I strongly suggest calling: Contract Glass in Wilmington (800-649-9957), they are a tempered glass manufacturer and have a oven on site. Most of the other glass shops in the surrounding areas get their tempered glass from Contract.