There are a ton of items in pots and chests where materials can be found for use on the forge. Smashing pot after pot gets tedious after a while so I'm wondering if I will ever come across shops that sell materials for gold instead of going area to area trying to find every chest and pot.
Vendors will not sell every single material, such as rare ones. However, vendors in towns can carry some of the more common materials. Additionally, the vendor in your campsite will often have the most common materials for sale.
Unfortunately, you cannot rely solely on vendors to craft items. You will have to go out and find materials in the wild.