Doing a Google search for cast polycarbonate tube gives several places to choose from. One site (usplastics.com) http://www.usplastic.com/catalog/produc ... 5Fid=24993
seems like a good start. They also have a "live chat" which may be useful. On one site they listed dozens of "trade names" from different manufactures. You may need to just write a standard set of questions/requests in word and e-mail them to a number of sites. I can very much understand the frustration of attempting to narrow the results as I have many times experienced the same. My primary field is electronics as I work for two television stations although it seems things have expanded to "ass gaskets" and "urinal mints".
My knowledge of plastics came from an "old timer" who retired and I wish I could ask him. So now this is what we would call "pick and shovel work". As for cost, again, the less expensive the better and it is a bit like buying oats. "If you wish for nice, clean, new oats this has one cost. If you wish oats after they have gone through the horse, these come at a much lower price!" I will keep looking for you. I understand you wish the material to be as clear, strong, and dimentionally stable as possible.