Most hardware or agriculture type feed stores (agropecuarias) have black rubber boots the country folk and farmers wear a lot, but haven't seen many white ones like worn in Louisiana at wharfs, for fishermen, etc.
Here in Higuey, I saw some in Iberia supermarket. I guess because a lot of hotel workers wear them. But I guess they would be sold elsewhere, too. If you won't find them elsewhere, I could get them for you and ship to you via Metro.