They seem like they'd be easy to make. But where would one find the right fabric to hold up outside?

Oh, craftybear - you read my mind and I just posted a question on a pattern.
Thank you. You are so active and terrific at coming up with the right response at the right moment.
Dodee

You can order it from Newark Dressmaker and other online stores. It's flag or banner nylon. Sometimes JoAnn's has it too. You have to either make a blank to cover the "seams" or else do both sides the same and sew them together. You can also applique and trim out the underneath fabric so it's the same on both sides. Or you place it so only the good side shows.