Richard,
It sounds like you are referring to the 55 Aquajet ARV series water pump. The pump is available from a number of sources.
Even though the pump will produce 5.3 GPM (gallons per minute), your showerhead will not allow that high of a flow rate, so there is no way that you will use 50 gallons of water for a shower. The 5.3 GPM flow rate is for unrestricted flow.
Gerald