If you're after a particular model of bike, then you just have to be patient and persistant.
Put the word out (particularly in the local VMX club & bike shops), check the many and assorted websites religiously, buy Just Bikes/Motorcycle Trader as soon as the come out, check local notice boards, etc etc.
I was speaking to someone recently who reckons he won't touch anything that's already been worked on because most of the time the work is below his standards, and he has to do it all again.
I don't entirely agree, but there is definitely value in rebuilding a bike yourself - at least that way you avoid the Curse Of The Previous Owner*.
If you're going to go racing, then the best bit of advice is to get along to some race meetings and introduce yourself. Once you're seen to be a real person (rather than just words on a PC screen), people will start telling you about bikes they know of, or giving you a phone call when they stumble across something in someone else's shed.
Most of the bikes I've got (and most of the bargains) have been found by/purchased from mates - although very few of them were bought 'ready to ride'.
*COTPO is when the previous owner has done rough/shoddy work. Of course, they'll get the blame for everything that ever goes wrong, even if they've never touched that part of the bike.
And everyone does stuff that's not 110% - usually because we're in a hurry or the parts aren't available or similar - but at least we know why there's a short-cut, whereas when you see a dodgy screw in your new pride and joy, you'll curse the previous owner for being a bodgy bastard...