Bob has the best price for this, but with shipping it's 15 dollars and you have to snail-mail him payment. There is a dude on ebay selling these for 15 dollars (and 6 shipping), which is too much IMO, but you get to use paypal. Those are the best options.
I have gone to radio shack before and they didn't sell the parts to make one of these, so I wouldn't bother.
I need one too, but have been too lazy to decide who I want to order from online.
It has always annoyed me to no end that in addition to the clunky sound board and cables on these old midway boards, they need a volume pot, too. Just adds insult to injury. I usually just grab my MK3 PCB over MK2 because I never got a sound pot with mine.