Dominican Coins
You can try Ponterio, a well-known coin dealer in the US. They hold auctions on a regular basis and sometimes you find nice early "proof" strikings (rather rare) from the '30's, and down to the late 19th century.
www.ponterio.com
Otherwise, anywhere you may look at coin shows, and other dealers... Yes eBay can be a good source. Sadly, very few old really collectible coins (in good condition) are to be found in DR itself. Looks like they have all left the country. Maybe a lack of local demand?
BTW on eBay you will often see those Santo Domingo 4 maravedis of Carlos-Juana (minted around 1535-1556), for good prices. Those are rather ugly, being hand-made, but they are reputed to be the first coins minted in the New World. The Santo Domingo mint started its operations around 1535, about the same time as Mexico.
Happy collecting