I would not replace the shot with a slug. The wad columns are different for pressure, and accuracy problems. What kind of gun is this going to be shot in? Is the bore rifled, or smooth, is there any choke. If the gun is rifled, You can get a mold, and cast your own. That might be easiest, as some sort of lead alloy should be available as scrap. If you give us a better idea on powders I thinke we can help.
G