Hi Lorenzo,
please don't take this the wrong way but are you sure you want to be owning a gun in the DR. Do you need to? Do you have anything to protect? I am not 100% sure on DR ownership but I would expect it to be a costly affair. Also the majority of people (expats especially) probably have something to protect, business and property maybe. Yes no doubt you would find guns on the street for little money but do you really want to get involved in that world before you even get into the country.
I can assure you I would never drink with a guy who owned a gun bought under the counter or off the street. I've worked with a lot of rednecks that make an empty bottle of beer a crazy weapon once drunk, god knows what would happen if they got drunk and had just come back from a war situation and a gun was available.
I expect they have gun shops but I wouldn't expect it to be cheap at all.
Good luck friend, seriously I wish you all the luck in the world, but leave your guns at war if you can.
Bill