1922 is a Peace dollar (a Morgan dollar is a different coin). One in decent condition will run you about twenty bucks. You'll need two of them.
When they are altered to make a two-sided gaff, one coin is machined into a shallow cup, with the outer rim and ridges left intact, but the inside of the reverse cut down to about half depth. The second coin is cut down to a thin wafer leaving only the obverse, less the outer rim. This piece is then epoxied into the shell created out of the first coin. The tolerance is very fine and leaving the rim intact helps hide the seam. If done well, they look pretty darned good. The biggest tell is the sound when it lands on a solid surface (kind of a "clunk" instead of a normal coin "ring"), so catch it in your hand if you flip one.
If you're the machinist, start with a clean workspace and save your shavings. You'll wind up with close to .8 ounce of scrap silver for each prop produced.