The Mythic Spire Jumpboots are a new addition to Fortnite in Chapter 2 Season 6, and it will take some work to get your hands on them. The first thing you will need to do is make your way to the Guardian of the Woods.
You can find this giant tower in the Weeping Woods, and it is protected by a Spire Guardian. These are quite tough, and will not be an easy fight. The Spire Guardian has a lot of shields, so be prepared for a protracted fight and make sure you are well-armed.
When you kill the Guardian it will drop a strange looking crystal that you can pick up. You can throw it, and while carrying it you will have an extended jump.
Now for the tough part, you need to get this to the Spire. Yes, that is not a joke. You will need to carry this, mid-game, all the way to the Spire. When you get there, about have way up is a shrine that you can place it in, and after a few seconds the Mythic Spire Jumpboots will fall out.
The Jumpboots will give you a massive jump when you chain three of them together, sending you high into the sky and allowing you to throw out your glider again. They can be very useful in gunfights, as even the first two jumps are much bigger than normal ones, so you can easily clear walls and surprise enemies.