HELP need seperator tank 1996 7.4 mercruiser
 

Or maybe how to buy a replacement pump on the inside

This is a perfect time to rid yourself of the VST all together. Get a high pressure fuel pump, out of the separator, run it through a filter if you like, to the fuel rail, through a regulator and back to the tank if possible or back to the water separator at least. You then do not need a VST and will not have any potential VST problems anymore like vapor lock.