There are indeed white Dominiques, I have some myself. (No, they are not for sale) In fact, the very first Dominique chick I ever hatched was white, which was pretty surprising to me at the time. There are records of white Dominiques popping up all the way back to the 20's. They are not a recognized color, and as such are considered culls or novelties, depending on how serious you are about your chickeneering.
Read more about them here: http://www.dominiquechicken.com/White_Dominiques.html
However, a breeder I know of whose line carries recessive white was incredibly generous sharing hatching eggs with several flocks across the country. While I doubt he would have deliberately used a bird carrying white in his breeding pens, (He raises them to butcher or culls them as baby chicks), still I suspect that within a few years, we will see them popping up here and there. Have patience.
From researching I believe the White Dom is what they call a "sport" hatched from Dom parents - the way Exchequer Leghorns were a "sport" of White Leghorns which were first documented in a Scottish flock.