So....when you cash a traveler's cheque or some US$ cash and get pesos, that is want you want, right? I don't understand why you think getting pesos to spend in a place that sells everything in pesos will be a problem. Just don't cash too much US$ at one time - if you are in an all inclusive, you may want to limit it to $20 or $50 ( to get about RD$1000 or RD$2500 at one time), so you are not holding pesos when you leave.
And if you are NOT in the DR yet, DON'T buy pesos at home from a bank etc., they sell them to you at terrible rates and there are places to buy pesos in the airport - in SDQ airport, as an example, before immigration/passport control and again in the luggage area before customs and again in the main terminal and then again in every town you will be in.