If you need a MAF sensor o-ring and you live in the US, head over to Home Depot. I found this faucet repair kit right next to the plain o-rings (none of which were the right size). It costs $10 and has 2 o-rings in it, sitting on the round plastic things, that appear to be exactly the same as my MAF sensor o-ring. The brand is Delta, and the part number is RP3614 A.

 

After having the O ring fall out in 2 pieces... no luck at the auto parts stores. The local dealers want $11 & $22, but neither have it in stock.

I found one at ACE hardware for $0.79. 😛 Has a nice snug fit... I put a light coat of Hylomar on it so it would slide in nicely. Even if I have to replace it every time I remove the MAF, I'm still way ahead.