http://www.ridoutplastics.com/20to275tube.html
They have both cast and extruded... I'm a cheap bastard and that link refers to the extruded. I've never ordered from them and currently I'm in the pricing out portion of watercooling... I have a wife and while understanding is not that understanding about dropping more money on my computer.
Here's the URL for the choice on extruded or cast if you want to get more pricing:
http://www.ridoutplastics.com/actubexcas.html
Pheaton's forums have a FAQ that they started on plexi that might be handy too as they reference a few more online stores in addition to some info on working with it:
http://www.pheatonforums.com/phpBB2/viewtopic.php?t=53