Eastward

Throughout the game you will have opportunities to expand your bomb and ammo capacities at merchant's such as 'Johnny's Emporium'

There are 2 upgrades to buy for each. The first one costs 3 gear parts and a small sum of salt, and the second one costs 3 advanced gears parts with another small sum of salt.

So, to complete this you will need 6 gear parts and 6 advanced gear parts which are found in the numerous treasure chests across the chapters.

I believe these upgrades can be bought at any time throughout the game, but it is likely you won't have the right amount of parts until the mid-late game anyway.