You do not need the hammer to synth a pumice, only two pumice pieces. There are a few pumice pieces in the game, the ones you'll probably want to go for are:
  • Spoil from beating Ark in Oeilvert.
  • One of the items in the Outer Island 2 chocograph treasure.
When you have the ingredients, beat Hades in Memoria and he'll synth it for you. You can also synth Save the Queen here for 50,000 gil, a javelin, and a pair of silver gloves. Both the hammer and Save the Queen sword are simply for bonus material in the ending. The only synthesis that consumes the hammer is the tin armor and the pumice piece.

If you want the hammer, find and give all the Stellazios to the woman in Treno and you'll be rewarded with it.

Here are the locations of the Stellazios:
  • Aries - Dali's Windmill.
  • Taurus - Behind Treno's item shop in the slums
  • Gemini - Throw 10 gil into the fountain at Treno's entrance 13 times.
  • Cancer - Behind the overturned cart near the entrance of Burmecia.
  • Leo - Near the statue of Neptune that transports the party to Alexandria Harbor.
  • Virgo - In Black Mage Village's Inn.
  • Libra - Right side of Madain Sari's fountain.
  • Scorpio - Below the HP/MP restoring spring in Quan's Dwelling.
  • Sagittarius - Left of the Gysahl Pickle cart in Lindblum (during reconstruction).
  • Capricorn - In the waters to the right of the entrance to Daguerreo.
  • Aquarius - Right hand chest at the entrance of Ipsen's Castle.
  • Pisces - Chest inside the Invincible airship.
  • Ophiuchus - Collect all 12, then search where Scorpio was found.