For someplace local, you could try Ferreteria Linares, right in town. Or if you can, go to Santiago and check out Ochoa and Plaza Lama, I believe they won't charge extra for delivery to Cabarete. The easiest and most efficient solution is probably split units in the rooms you want a/c (as opposed to central A/C as you would probably do in the US)...and most places include installation in the list price.
There's also a place in Puerto Plata (right hand side, shortly before the Burger King roundabout) I think it's called Refripartes or something like that - they installed a few for me a while back; good work.
try Delio Polanco he has install most of all the AC in Cabarete and uses TMG which parts can be found all around town. Great guy work for me for more the 10 years, great service just a phone call away.