I found an air leak around the choke plunger on my 81 XT 250. Does anyone know where I can find a new plunger? Looked online at OEM parts fiche and nothing available. Seen one on eBay that said it fits most VM series carbs. Not sure about it though. 

That's be a very unlikely carry on. Curious to know how you found that out. Call me nosey--just don't call me late for dinner. I'm pretty familiar with those carbs but not for a while i must say. Just a normal plunger but think they had a needle in the middle for a controlled idle speed. It'll have a rubber pad built in as well to seal when choke is closed for normal running.

I just noticed when I’d rev it the rpms would slightly hang before returning to idle. So I started spraying things with carb cleaner and noticed the idle changed when I spray around the choke plunger.

Is it possible the spray was getting around the inlet manifold rather than affecting the choke. Hold finger pressure on the choke to see if it alters. Those inlet manifolds had a habit of coming unbonded & coroding under the bonding.

So I think you may be onto something there with the intake boot. If I spray down along the shock in the direction of the intake boot where it comes out of the air box the engine dogs down and about dies. I found a new boot. Ordered it. I can feel air escaping around the airbox too when I rev it. Or so it seems like. Is it possible air leaks around the box and that boot could be leaning it out a lot? 

No sorry, i was talking about the manifold from carb to engine as they come apart. If it was that bad you'd be able to move the carb about & see it pulling away around the alloy flange at the head. Well, air filter hose might resist letting the carb move. Wish i could find a pic. Last one i did was a Kodiak 400 quad & same deal, just grew corrosion where rubber is bonded to the alloy. Bit like brake linings letting go with corrosion. Not saying that's your problem--but just guessing.