Here are some direct links where you can find Steam Wallet Cards:
Steam Wallet Card - $20 Steam Gift Cards - $200
Some Steam Gift Cards are sold by Valve directly (the company behind Steam) at better prices but for most of the year, are unavailable for sale. Click on this link to see if they’re back on sale again: Steam Gift Card - $20.
Scratch the code for your Steam gift card
How to redeem the Steam Wallet Card or Gift Card
If you have received a physical Steam Wallet Card or a digital Steam Gift Card, copy the corresponding code from it. Then, open the Steam app on your PC, login and go to “Games -> Redeem A Steam Wallet Code.”
Go to Games > Redeem a Steam wallet code Paste or type the code on the redeem page, and click or tap Continue to add it to your Steam account.
Enter your Steam wallet code The Steam Gift Card or Wallet credit is added to your Steam account. If you never used Steam credit on your account before, you may be prompted to enter your address to determine the correct currency. Once all is done, you can purchase any game on Steam.
But wait, where can I buy digital Steam Gift Cards?
Buying digital gift cards is done directly from Steam To buy a digital Steam Gift Card, go to this page on Steam: Digital Gift Cards.
Look through the list of items you ordered, find the Steam game that interests you, and look for the Activation Code or Activation Key highlighted below. Copy this activation key and open the Steam app on your PC.
Copy the activation code for your Steam game Go to “Games -> Activate a Product on Steam.”
Go to Games > Activate a Product on Steam The Steam product activation wizard opens, informing you that you need a product code. Click or tap Next.
The Steam Product Activation wizard
Enter the Activation Code/Key for your game