I bought mine from a guy in Singapore on eBay. Aquatic Magic would be a good eBay source also, I believe he's in Malaysia. Stateside sellers don't seem to have caught on yet, or possibly the import costs just don't allow them to be competitive. You could always go with the genuine ADA version, but it'll cost quite a bit more than eBay.
It doesn't have to be glass, though they're more attractive that way. Red Sea makes one out of plastic, and I know a member here used to make and sell them.