Yes, there are many ATMs in San all the banks and also freestanding in several locations. We do our banking at Scotiabank both in Canada and Belize so we don't pay any fees but you will likely be charged between $3 and $5 per transaction. Because of that, I advise that you take the full amount allowed in order to keep the fees to a minimum.
Bring a few hundred $US cash(if you can, get a couple of $1s (for tips)and $5s (beer, taxis, snacks)) to get you started and you'll hit the ground running.

Newfoundlanders are the only people in heaven who want to go home.