Generally, it's best and easiest for vendors if you pay cash!
There are quite a few ATM options between Metro and the Party:
ATM at 7-11 (not sure of their affiliation)
Bank of America 2 blocks from the Takoma metro
Suntrst near intersection of Carroll Ave & Laurel Ave
ATM at Takoma metro is Chevy Chase (this might not be there! our local scout didn't see it)
A few vendors may also take personal checks - I know I do! So come prepared!!
A few vendors do take credit cards, so if you do run out of cash, you may still be covered! ;)