Regular twin bed sheets will work, though the fitted bottom doesn't fit the radius as tightly as you might like. We ordered ours from this outfit, they already have the dimensions, and they fit the Airstream twin. I believe they are/were the provider of these sheets for Airstream. They're nice sheets, high thread-count, though not cheap. I don't know about their delivery time, but you can call and ask.
Custom Recreational Specialties