To upgrade your Smithy, you need Smelting Anvils.
With each upgrade, you need more Smelting Anvils.
Below is a list of possible ways to gain these.
Gems
You can purchase Smelting Anvils using gems.
1 Smelting Anvil = 15 Gems
10 Smelting Anvils = 125 Gems
100 Smelting Anvils = 1,154 Gems
1000 Smelting Anvils = 10,000 Gems
Campaign Elite Sweeps
You can sweet elite campaigns to pick up extra Smelting Anvils.
Elite 3-1
Elite 3-2
Elite 3-3
Elite 3-4
Elite 3-5
Elite 3-6
Chests
Chest from the VIP daily clan gifts (Flamehammer city) gives Smelting Anvils too.
VIP Daily Clan Gifts
You can collect 5 Smelting Anvils per gift from daily VIP Clan gifts (Once you have level 25 diplomacy for this city).
Regular Events
Every 3 hours, you can do a regular event (Have to complete all 3 stages of it).
You gain 20 Smelting Anvils each time you complete it.