I think the gas cap sizes are pretty universal, you should be able to get one at WalMart, or Autozone, or Advanced... Something like that.
... or I could be making stuff up, but I'm pretty sure you should be able to find something.
'86 325es (TURTL3): PJ peetsa hawler
'87 535is (BADWATER): almost done... really.
'86 325e (GRN HLL): DD/show/rocket
'04 x3 3.0i: Wife's Bimmer (manual, woot!) My BMW Blog