There are videos on this topic on YouTube. Search for "figuring skating boots plugging holes" or something. I skimmed some of them, because I was curious. Apparently you have to match the sole material with the plug material. So a leather sole requires a leather plug. And you get leather material from a number of sources. A wooden sole requires a wooden plug. A composite sole requires a composite plug. Edea sells its own plugs for its boots. Probably most of the major boot manufacturers sell them as well. But you don't need to buy anything specifically for boots. You can use raw material that you can get anywhere. The videos explain it.

Once you get the plugs, you have to clean up the holes by picking at them and sanding them flat. Then add glue or epoxy to the holes. Then insert a plug and pack it down with a metal hole punch. If you need more than one plug in the hole, add glue again and put another plug into the hole, push it down with the metal hole punch. When the hole is packed solid, let the glue dry 24 hours. Then sand the outside of the hole to get rid of the glue residue. Sand also the rest of the sole to remove the wax layer and any dirt. Then add a wax finish. At that point, you're ready to drill new holes.

This is what I get from watching those videos. The exact details probably vary from person to person, each with their own preferred techniques and materials.