Most of the shops in DR have the prices in both US and pesos. I used to always pay in US $ until I went with my sister one time and she uses pesos. She was consistently getting better prices from the vendors, the resort gift shops were set prices.
There is usually a foreign exchange booth on every corner in every little town with a chalk board outside with the rates of exchange. They aren't all the same so shop around. The Canadian dollar isn't as good as the US for changing.
You don't have to change your money into pesos, everybody will accept US, it's up to you.
