Can I use 12AWG for my grounding pigtails of 14AWG?
Assuming you mean pigtails for use inside an electrical box, yes using 12AWG is fine. 12 is heavier gauge than 14, so generally wherever you use 14 you can use 12.

But you can also just make your own pigtails using a cut of wire from your roll of 14. It's not necessary to buy pre-cut pigtails. And since you can use 14 gauge you probably should, especially for pigtails where the lighter gauge will be easier to fold and tuck into the box.