Once you're able to chat with her about the boots, Avita will tell you that you must prove yourself worthy before you can acquire 'the holy relic Kynareth has bestowed upon the world.' You're to travel to the Grove of Trials, where you will be tested by Kynareth herself. There is a bug that can keep this dialogue choice from showing up, but you can circumvent it by making sure your Knights of the Nine mod is the last one to load when you fire up the game. When you reach the shrine, strike up a conversation with Avita Vesnia and ask her about the Boots of the Crusader.
There are no other locations in the immediate vicinity, but fast-traveling to Fort Nikel, Breakneck Cave, Fort Wooden Hand, Haynote Cave, or the Fanacasecul Ayleid ruin will get you pretty close. Your compass points you to an unmarked location in an otherwise unremarkable wilderness area. If you want to follow in his footsteps, Sir Juncan instructs you to speak with the priests at a Shrine to Kynareth in the Great Forest west of the Imperial City. When you speak with Sir Juncan, he'll tell you that he knows the location of the Boots of the Crusader, but was killed before he was able to obtain them. After retrieving the Cuirass of the Crusader during the Priory of the Nine quest, you'll need to speak with all of the ghostly knights to learn more about the whereabouts of the remaining artifacts.
