I found a local place in the Yellow pages under "plastics" . It's on a small industrial estate and has sheets of the stuff that it sells to local industry and sign makers. I snaffled a few offcuts (with their blessing ) from their skip and it cost zero.
Chris is right, Yellow Pages, there's a plastic stockholder in most areas. Tufnol is the material of choice but very smelly to machine. I would go with the polycarbonate if you have the choice (you will)
If you are using for a router table plate, Bill Hylton in 'Router Magic' says that polycarbonate is not maybe as 'sag resistant' as acrylic or phenolic if you are hanging a big router from it. Having said that, I bought a piece of polycarbonate from Rockler when I was on holiday last. Haven't bought the 1/2" router yet though!
Anybody got any experience of polycarbonate for router plates?
Tough as old boots but I don't reckon PC is stiff enough. Polypropylene (PP) is fairly stiff and reasonably low friction. Glass filled Nylon even stiffer. I'd go for Phenolic Fabric e.g Tufnol if you can afford it.