Is it available as a simple flat sheet anywhere? I'm looking into making a prototype diffuser and just need something to get the basic shape going, then adding & hacking it around accordingly.

Any suppliers out there?

I found a sheet of melamine, some 600gm weave from CFS and cheap resin makes some pretty quickly...

It's used in the commercial body building trade and available from companies such as Polyfibre in Birmingham.
http://www.polyfibre.co.uk/product.asp
But as BenB has said, you can make a simple inexpensive buck from melamine faced chipboard and hardboard and make your own pre-moulded panels.

[Edited on 21/5/07 by Phil.J]

And the melamine in my garage is so covered in dust and oil spray that I don't need a release agent

Just look up any industrial laminating company in the book. They will knock you up a flat sheet for peanuts... Alternatively, have a go yourself!

As Phil.J says Polyfibre in Birmingham. Ask for Mike, he is most helpful. They do carbon fibre, GRP, Epoxy, polyester the lot. I use them for my boat stuff. I can collect if you get stuck.