Hello,
I had the same questions about the availability of parts for the 250R about this time last year. I have an '88 that I've been lugging around for the last 13yrs. I started racing again this year. I've had no problems finding OEM parts and after market parts for the 250R. My motor was originally done by Duncan racing in the mid 90's and they are still making all the motor kits and after market stuff that was available then. I've been looking at Eddie Sanders Racing as well for after market parts. As far as OEM goes Service Honda sells their Honda parts at 30% off list price. They have all the 250R schematics on their web site and you can still get just about any OEM part.
Service Honda Home of the CR500AF 500AF 500AFX Junior*X * Honda Motorcycle Parts
If I were going to buy an R I would get an '88 or '89.
There's a guy at the local track that had a 250R, and he was beating all the fourstrokes. For some reason he decided to sell it and bought a brand new Yahama YFZ 450. He's had nothing but trouble with it all season and he's already sold it. Word is he's looking for another R. In my opinion there's still nothing as stable as a 250R as far as motocross goes.