Many years ago I switched out my mechanical pump for an electric on another vehicle. I simply got a piece of scrap 3/8" aluminum plate(thickness not too important), used the gasket for the old pump as a pattern and with a hacksaw and file made my own. There may be a metal recycling yard near you or a small quantity metal dealer or even a metal shop that will sell you a small piece of scrap.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles