Unless you lost the idle screw, that probably isn't what is stopping it from idling. For mikuni parts, try Sudco ( www.sudco.com ). You typically don't change an idle screw to improve idling, though. If you still have your idle screw, I'd start by taking the carb apart and cleaning it out thoroughly, including all the jets.
What a PIA. A lot of MX bikes were never designed to idle. That way, when you wreck and your bike crashes through the fence, into the crowd, the engine shuts down and no one's hair gets caught in a turning sprocket. You can sometimes cheat by tightening the throttle cable.