Uncut steerer tubes too short for 21" Frames
When I have had this problem I have paid to have extensions welded on to the tube. The welding must be done in a lathe, or a proper rotating pipe welding jig, so it's not a DIY project. Or look for a fork with a removable steerer tube and fit a longer one.

One advantage you have here is that many (most?) steerer tubes are still steel, so it's usually possible to weld them without destroying them. In my case I've used fairly cheap forks, so even if one gets trashed during the process I'm only out $100 or so. Also, with a little work you end up with a tube stronger than the original because you can sleeve a second tube inside the original and weld that at the fork crown too. Or if it's a tight fit, just leave it there.

For my bikes there steerer lengths required have been unreasonable (in the worse case, about two metres for a tall bike), so there was never a question of being able to find one pre-made.