These aren't related to the North American Hogs at all aside from their common name. I recall reading somewhere that they were actually more closely related to Naja than they are to Heterodon, but don't quote me on that.
You typically don't see many for sale, but I'm pretty positive there are a lot more than 25 around. Especially considering that it is not unheard of for them to have single clutches that large.
Pretty sure there was a seller on KS not long ago that had four breeder pairs for sale. Not sure if he still has them, though.