Stay away from street vendors. They may give you 5 more points than the banks or exchange houses, but you run the risk of losing it all from a vendor with a slick hand. Normally, exchange houses have better rates than commercial rates. The exchange houses will usually give you a better rate than a business that sells in pesos. With the depreciation of the peso continuing, more and more businesses are charging in US$, so many operators may ask you to pay in this currency. They will accept travel checks or cash.
See
http://dr1.com/travel/prepare/currency.shtml for more information on currency operations in the DR.