Hi Trip, I agree with Louey - acrylic works just as well as glass (better at times).
You can get it from virtually any glass shop, though you will most likely have to pay a minimum cut fee (eg. when I needed to build a small baffle in a terrarium to create a water area for my frogs, it cost me $10 even though a piece twice that size would also cost $10). Call ahead and get quotes.
I'd suggest the thinnest acrylic available for your top, simply because that's less solid material the light needs to pass through. I think it comes as thin as 1/16" - that should be fine unless the piece is substantially large.
I have also seen sheets of acrylic at Home Depot (and Rona in Canada - Lowes most certainly has it as well) and it's quite thin - I believe they sell it as a window cover to make the glass tougher to break or something.
Like Louey said, though, if it's going to get substantially warm, go with glass instead as the glass won't discolor (it will of course shatter if it gets TOO warm and you splash colder water on it).