Looking for rear wheel options for the 09 TE510

I'd like to have a second rear wheel fitted with an MX tire for those times/conditions when that tire might work better. Does anyone have any ideas for what will fit (maybe from other bikes to make searching easier) or maybe someone has a used one they'd sell? I'll end up putting all the hardware on it so it would be a simple swap so if you have one ready to go let me know.