I think Micro-Tools might have felt and copper comb for light seals. I know I've seen it somewhere.
Bookbinding tape is also suitable for the flaps. Remember the tape doesn't need to be lightproof and usually isn't. The tape just holds the flaps in place, and the flap forms a light seal by meshing with the body of the holder. You can find bookbinding supplies at www.talas-nyc.com