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.
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.