Scattered around the entire map are buidings that contain a workbench which you can use to replenish lost supplies assuming you have the materials needed to craft them. There are 8 workbenches in the Volga and 3 in the Caspian Sea. You will find these building when you do the main or side missions.
At workbenches you can do the following:
• Craft a new weapon
• Craft ammo
• Upgrade weapons
• Craft medkits
• Upgrade equipment
The Volga
The Caspian Sea