I am fitting a Beseler head to my durst 184, and it seems I should have a sheet of plastic between the cold light tubes and the negative carrier. Any ideas of a good source? i spent a long while on google and did not score.

thanks

Sabic, the successor(?) to Caddilac Plastic & Chemical:

http://www.gepolymershapes.com/polyshapes/en/Home/Home/home.html

They have a will-call counter at their branches.

I am sure they have it, the problem is finding out what they call it.
Try calling the local branch.

U.S. Plastic is a good source. I ordered sheets of 'delrin' plastic from them for my frankensteined 4x5 do-it-yourself enlarger. I also tried other types of plastic (acetal, polystyrene) from them and found that any white colored plastic will do -- all will be translucent as long as it's thin enough. I like the delrin as it's more rigid and less prone to be warped when you get it.

McMaster-Carr, http://www.mcmaster.com is also good for a lot of materials.
I also second U.S. Plastic. Their mail catalogs are good to browse and offer a lot of (pricey) items that can be useful.

You could try a sign making company in your area. Ask for "sign white " acrylic sheet. A plastics supplier should also be able to sell you a small piece.

I bought some acrylic for a light box from a local window glass place.

Is this a great site or what? I got one from a sign shop, custom cut to 14.5 in square, all for $15.00.

Halleluja