You may not need to replace them. You just need to get the brand off the valve and locate new gaskets. If they need to be replaced, a Valterra valve will work as a replacement. While our rig was still under warranty, we had the black valve break, and Camping World near Boise, Idaho replaced it with the Valterra valve. Hope you can get the black tank valve to open before working on it, as this will be a bad experience for you if you can't.
http://www.valterra.com/product/wast.../?pcat_id=1445