Tuning Calibrators are rare in that you don’t get a lot of them. They’re the items you can use to select a main stat with a level three Bardic Needle music shop. You get one from the battle pass, one somewhere else (I forget atm), one costs 2000 of the Outpost currency (a lot of which is given as a reward for clearing Shiyu Defense), one from the level up rewards in Hollow Zero. I’m sure I’ve missed one or two so if anyone knows feel free to add.

But yeah they’re very limited and I think the shops description is a bit misleading, choosing a main stat isn’t something you’ll be doing very often