Almost any small hardware store here has "chair tips" or "crutch tips" that slip on over the outside of a tube and "close the hole." The chair tips usually are plastic, with a smooth (slick) end, while crutch tips generally are rubber and have a "tread pattern" so they won't slip on the ground in use.
Plugs to slip inside the tube and close the hole up are a little harder to find, possibly because for a best fit you really want to match both the ID (so it will stay in) and the OD so that you don't have a rim to catch on things and pull it out; but should be available for common tube sizes if you look around.
You just have to keep track of where the best-stocked shops are in your area.