Re:
It's possible they're not available as a spare, if it is a bush and not a bearing.
Probably not the answer you were hoping for, but I have the rigid frame version FS730, 18.5" i think, which i keep meaning to build but never get round to it, i'd let it go for £20, if you're interested. (Postage TBC) Everything should swap over easy enough. I also have some matching Rst forks and seatpost to fit.