Brian,
I just got back from the local BMW motorcycle dealer and he said the same thing! Go check-out the motorcycle salvage yard!
The new BMW motorcycle caps are $85 (new) They look nice on the outside but since they had none in stock I'm still wondering about fittment. I'll try the salvage yard as you have suggested. Only caveat is I'll still need the services of a locksmith or replacement tumbler from BMW or Both to get it lockable, I reckon. I may need to have the top of the filler neck welded on to the receiver or bottom piece of the cap assembly in order for it to work.
David