IMHO,
Just go to an ATM once with a driver you trust. Get as much JA as you need for the visit. There is no better exchange rate but you may pay a fee to your bank or the ATM. Either way it's as good as you'll get.
I go to one that is very little used and never have to wait or deal with anyone hanging out.
Put it in your room safe and be your own cambio. If you have any left over(LOL) save it for next time..
