I had a similar problem Nicky, one of the ceramic guides popped out on my barbel rod.

The local tackle shop said they don't come seperately but it only cost me a fiver for him to whip a whole new ring on for me.

It's very easy to put on new guides yourself, so replacing the whole thing isn't really a problem...

There are some very ilustrative guides free on the web and even one here on FM!!! And the library is sure to have at least a few good books on the subject...

So my advice to go out and get some new guides and start whipping! A lot of times the and product is a lot nicer then the fabricated ones because after all, you are working on your own baby!