I have had good luck on ebay. I got a stock full rear wheel/hub/tire assembly for my 03 KX250 for $135 (+$20 shipping). It had one broken spoke that I fixed and the PO had done a horrible black spray paint job on it. He didn't even bother to tape the spokes so they had over spray all over them.
I could care less though. I don't have to swap tires when I want to run a paddle.