Brugal Siglo de Oro can be difficult to find, but it's available in bigger liquor stores (12 year old ultra premium) in the blue porcellain bottle, and Licor Unico (30 years) in a red porcellain bottle.
BTW, you can tour the Brugal factory in Puerto Plata for free - and buy all their available products at a great price right there.
Bermudez's factory is in Santiago - they also offer tours, and I assume, a company store. Not sure how easy their premiums are to find, but I believe their top-shelf version is Bermudez 1852 Anniversario (12 years),
Barcelo' is in Santo Domingo, Im not sure if they offer tours, but I assume so.
Their premiums are pretty easy to find: Barcelo Imperial and Barcelo Gran Anejo.
I havent tried it, or heard much about it, but you may want to seek out Matusalem... a "Cuban" rum distilled in Santiago. Look for Matusalem Gran Reserva...