If you look on the "special order" pamphlet attached to the rack at Home Depot, it's in there.
It's REALLY expensive, but it's in there.
You're probably going to find the best deal by looking in plastics in the Yellow Pages.
Most cities will have a few dealers that sell to the local sign shops, etc...
You are going to get the best price that way; and can probably get a good deal on having it cut, provided you don't want anything fancy done with the cuts.