<A HREF="http://www.bancentral.gov.do">http://www.bancentral.gov.do</A>/
The official exchange rate is found here. market rates are slightly better, within one peso to the dollar; the best rates are found on the street or at small independent "casa de cambios" but you should know what you are doing before using someone on the street.
The banks will usually give you an ok rate, at least better than the hotels. For actual market exchange rates surveyed daily, go to
<A HREF="http://hoy.com.do/agosto99/030899/moneda.htm">http://hoy.com.do/agosto99/030899/moneda.htm</A>
but bookmark the site, not that page, as they update the site everyday. Follow the "monedas" link