An original synthesis of the cited guides. Adjust the approach to your difficulty and equipment.
Find the right vendor
Visit Father’s Forge at home for melee weapons, Ran for bows, Ina for bombs, and Murata for firearms. Look for a separate menu for carrying-capacity upgrades.
Plan by material region
Remember Ishikari for Oni Raider Mask Pieces, Teshio for Shinobi Steel, and Oshima for Gun Parts when planning your material runs. Gun Parts are also used in sword and bow upgrades.