Exp bands can be obtained by the Signet NPC in the Chocobo Circuit, and recharged there as well, as often as needed.

Alternatively you can talk to the NPC at the exit of your nation and buy one for 2000 conquest points under "common items".

You may only purchase or recharge one EXP Band (Chariot Band, Empress Band or Emperor Band) once per conquest tally.

The Empress Band will net the greatest amount of bonus EXP on SuperNova