Hi Gwijayas
You can find the NPC Bale's Power-UP Potion Seller right in front of the castle gates at ELIM D5 between the NPC Soldiers and next to NPC Noble Madam .
About the Vouchers for 100 Bale's Power Up Potion's can be found only by Killing Bosses at Underground Waterway or Sage Tower Bosses.
Those can be exchanged for 100 Power Up Potions with the mentiobned NPC .
Seal Online Team