You can save before approaching or trading with merchants each time you go up a level, and the reload a few times to see if they have anything useful.
After you have the battle tower, find at least 1 of each type of zeppelin parts in or near wyvern nests in the fjords, and talk to the zeppelin master on the harbour building to get to Aleroth, where the jewellery merchant would be one of the better ones to check for formula (the one in the bunker in the fjords can also have formula).
