Myself and a couple of other riders used to go to this Shop in Santiago (can't remember name) that was located on the right hand side just before the entry on the Mountain Road that takes you to Sosua. There is also a crew of ATV riders in Santiago and I believe they have a website.
I found some tires at Comercial Santana in SD, at better prices than in the US. I wonder what the catch is, maybe they left the peso price after purchasing the tires years ago, but I doubt it.