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.