Hi Pete
Greetings from Brisbane
I inherited an MC21 a couple of years ago and have had no issues fining any part. Ebay is your best friend - there are a couple of guys in Japan that regularly break NSR250s and list them. Be patient and don't try to make it your every day bike. The hardest thing I had to find was a PGM and now I have 4...........
My biggest tip for parts would be to use the parts diagrams on the site and make sure you know the nuances of what you are after.
Cheers
PK