Also you can order money from your bank and they will mail it to your house. I don't remember the exchange rate. It was easy so you don't have to worry about it when you get down there unless you run out
When I'm in a Dominican Republic (and that's 150 days a year) I don't take cash with me. I bring my ATM card and just pull it out the machine. You get the best rates, you don't have to worry about some casa de cambio robbing you on the exchange rate and you don't have to carry cash. Just make sure you tell your bank that you're traveling so the ATM there won't swallow your card.