By going to Ebay and searching on Yamaha fuel cap, I found 2 quickly (gulp they were $30 plus shipping). I caution you to beware of the two sizes.
For the older TWs 2002 and before (and the old xt350 tanks) you need the large one. The same design was used for smaller caps (such as fit the newer TWs) so some of these may get confused or the size not specified. I have posted photos of the two sizes elsewhere (see the xt350 tank install) but it warrrants repetition with new forum members coming on all the time. For the record, I no longer have these caps available (actually I have one of the smaller ones) but they should be possible to find with diligence in dust bins of MC shops or salvage yards. Tom