Domincam rum is one of the best in the world, like anything else there are many brands and prices.

So called airport duty free stores are much more expensive than purchasing at a local liquor store or supermercado in the DR.

North Coast
In Sosua at Super Super Liquors at the corner of Pedro Clisante and Ayuntamiento streets has the best prices in town and I would recommend my favourite, Brugal Extra Viejo or a local favourite Siboney Red Label.
POP right at the Brugal Rum factory as you go into town.

East Coast
Super mercado in Cortecito.

South Coast
San Pedro de Macoris, right at the Macoris Rum factory.