I can get $JA from my bank... I asked about it before our last trip and found that their rates were worse than at MBJ. I bring a debit card and take $JA as needed (usually from the Times Square ATM), and find that since my bank refunds ATM fees I get a better exchange rate than at the cambios.

I also bring a credit card that stays in the safe for emergencies... thankfully we've never needed it.