Why hold just 99 coins when you could hold 999 coins instead?
Introduction
There are 9 wallet upgrades hidden in the world. Each one will let you carry an additional 100 coins, up to a maximum of 999.
Wallet Upgrade Locations
These are numbered roughly in the order that you will be able to find them.

Wallet #1

Wallet #2
On the way to the elf village, you’ll see these brambles to the north. Clear them with Fire, then approach the chest and clear the brambles around it too. Inside the chest is another wallet upgrade.


Wallet #3
In the top-right corner of the elf village are some soil spots. Plant a vine and climb it.

You’ll be able to explore a small portion of the wind region. Which we’ll use to get to a small part of the fire region.
Here is the path you must take:

In short: go as far to the left as you can, past the patrolling soldiers (they can only be damaged from behind, and the enemies here might be tougher than you’re used to, so it may be best to avoid them.)
Go all the way left and cross into the fire region. Go down the stairs and open the chest for another upgrade.

Wallet #4
Go to this cave, which is south of where you can buy the ice map.

Proceed through, and solve the column-pushing puzzle so that all 3 switches are pressed. Go south and you’ll emerge near a chest. Open the chest to get the upgrade.

Wallet #5
This chest is out in the open, but you’ll need to reach its island via the bridge to the south to access it. It contains another wallet.

Wallet #6
You might recall, from the very beginning of the game, a house that was on fire, south of the fire village. Return there and equip Water, and use it to extinguish the flames in the doorway.

Enter the house and open the chest for a larger wallet.

Wallet #7
In the Earth Temple, head to this room on the 2nd floor.

Then take the staircase *down*. In this room, equip Water and travel clockwise around the room, avoiding the traps. Equip the Sapphire jewel if you have it so you can take an additional hit.

Wallet #8
In the very bottom-left corner of the run-down part of the electric area town, there’s a building on fire. Equip Water and put out the fire in the entrance to go inside.

Once in, keep Water equipped and make your way to the chest in the corner. It contains another wallet upgrade.

Wallet #9
On your way to the Wind Temple, you’ll pass by this cave.

Inside is a hidden staircase beneath a bush. Chop it down and go downstairs.

You’ll encounter a large room with a challenge: light all four torches. There are two moving platforms which circle around to the torches. But there are also huge wind gusts that will knock you off. You must hold the Iron Ball when the platforms pass through wind. But take it off and use Fire to light the torches and then quickly re-equip it. Once all four are lit, head right.
This room has four chests. Three are crammed with coins. The fourth contains the final wallet upgrade.

With the final upgrade, your wallet should be able to hold up to 999 coins!