I think Timbo may have a bunch of the tusk ones yet. They are not perfect but if you use a dab of grease on the threads they work fine. I may have a new one in the bag as well.
Any cap for a xr50 or crf 50 will work just fine. I bought one off ebay for $8 and it screws on just fine. Came with a nice thick rubber gasket in it also. If you want a fancy lookin one they have those to for about $50. But for $8 they have all the standard colors you would want.