These items are purely random.
Shopekeepers inventory resets after a certain number of days, whether that is 3, 7 or 30 is not known to me. However, I have noticed that if I reload a game, the shopkeepers inventory usually resets and the mix of enchanted items changes. So, you could just try reloading that save and talking to the shopkeeper. However, until the game decides to put one of them in there, it won't be there and there is nothing as a player you can do to influence it.
And, a question like this belongs in Spoilers, so I will move this now.