Vimenca has a branch in the pharmacy in the San Juan Shopping Centre in Bavaro. There is also one in Punta Cana Village. Vimenca and the savings and loans banks like Ademi and La Nacional usually have slightly better exchange rates than the banks.
I use Cambio de divisas next to Solo Pollo and Orange kiosk, off Avenida Espana. (Almost to Friusa) Not sure what you mean by larger transaction but 2kUS is no problem.