You should rarely ever need to buy items from an NPC for synthesis. You can make a bronze ingot using 2 copper ore and 1 tin ore. You can get tin ore from mining in Bentbranch or Bearded Rock, and copper ore from mining in Black Brush. Also, you can buy items off of the market wards in town if you cannot buy them yourself. Do note that all three towns have different merchant houses dedicated to them, so you'll want to search all three if you want to find the best deals.

Also, you may wish to look into local leves. You can accept these at the leve counter in the adventurers' guild. They allow you to do synths for free, and then they reward you with shards and materials you can use to craft on your own.