You can find punches sold through several suppliers, for the various gauges. See Dixie Gun Works, for one. And you can get the caliber/gauge information here on the forum. Go to Member Resources towards the top of the index page, then scroll down to " Articles, Charts, and Links."
The links takes you to the names of lots of suppliers, while the charts takes you to a number of different charts, including the one you need. Click on the name of the supplier, and you will bring up its website, and catalog.
Harbor Freight sells cheap punch sets, that go up to 1/2". Most shotguns are going to be 28, 24, 20, 16, 14, 12, and 10 gauges, for MLers. A 3/4" diameter ( inside) iron water pipe can be made into a good punch for a 12 gauge shotgun. A 5/8 Inside diameter water pipe works for the 20 gauge. The other sizes are made from other sizes of pipes found for other uses, or for most of us, its less a hassle to buy the punch for the less common sized barrels.
Before going to the trouble of making your own wads, price them through the suppliers. Most wads are being made by Butler Creek enterprises. Some are made by the successor company to Ox Yoke( formerly located in Maine.) Unless you are going to do a lot of target shooting, its probably less trouble for you to simply buy the wads, and cards.