Reincarnate that old tee-shirt you can't bear to part with into a new pair of undies.
Do-it-yourself web site Supernaturale has instructions on sewing a teeshirt - like, say, your collection of hair band tees - into a pair of briefs. Don't you think your bum would look great sporting a picture of Poison or Warrant on it? I do.
Tee-shirt underwear [Supernaturale via Gizmodo]