I thought you were talking about using that for the entire outside skin, not just the rounded corners.
This is the place I'm thinking of. They dont list the product on their site.
Plastic Supply of Maine
If it's just small pieces, I could probably snag them on ebay cheap enough. Aluminum vent pipe is what I was thinking of going with, but it's really too thin to hold it's shape on it's own. I was thinking of putting something in behind it as a backer....
I'm from Augusta, ME. There are a couple of sign shops near by. I'll have to make some calls when I get ready to do those corners.