I don't know of any in Santiago.
In Santo Domingo, around Pena Battle 1 block east of M. Gomez. There are several professional shops and you see a lot of cars being converted daily. There may be 5-6 shops in one block radius there.
Easy get - in: right after Texaco gas station make right going north on M.Gomez, and first street make left, and go 1 block north, on that street there are the shops.
It takes 4-6 hours.
I got mine at Leo Gas. It is a regular LPG gas system, it cost 15.500 pesos, got it 6 months ago, 8 months guarantee, works well, required 3 fine-tunes, I get about 33%-40% savings against gasoline moneywise, also depends on where one gets the tank filled, here in Higuey I do 230 miles at 15 gallon full fillup (into a 16 gallon tank!!!) on highway speeds (around 50-60 mph/80-90 kmph), have 2.4l 6V engine, gas system is Italian one, has electronic control inside, tank is a quality tank volume 16 gallon. However it is not controlled - gas- injection - system (similar to fuel injection)(- my father got that one in Europe - ) those can be done here as well, price is about 50-60.000 pesos.