They're not *super* rare, but it took me a little while to find a nice one out there. There have been a few on eBay within the last week. That's about the best suggestion I have. Best of luck!
I don't have the R but I thought the difference was just the extra jack socket. The OD-2 was made for ages so there's probably minor differences made along the way.
I had someone tell me that the circuit difference is more than just the additional jack, but I haven't been able to compare them. Hopefully I can pick one up soon.