You don't want to get that shipped. I work at a place that does glass (mostly mirrors). Any glass shop can get that for you, although only a few actually stock the stuff. Just open the phone book and start calling places and getting prices. You want a piece of 30x30 1/4" smoked tempered glass. If you get a choice you want the lighter tint.
You don't HAVE to get tempered, but it is nice to have, as it is much less likely to break.
The reason you want to call around is that any shop will sell it to you, but the few that actually do the stuff are going to be way cheaper on it.