How to buy diamonds without credit card

Aside from credit cards, the Google Play store offers alternative payment methods in many countries:
Gift Cards and Promotional Codes
Reference:

https://support.google.com/googleplay/answer/3422734?vid=1-635786710867633484-317308837

Please Note:
1. Gift Cards can be purchased at supermarkets, bookstores, newstands, etc.

2. Scratch off the back of the card to expose the promotional code.

3. Enter the game and tap the _$_ symbol next to the diamond icon at the top of the screen. There you will find gift packs and their prices.
Remember, the price of the pack you wish to purchase cannot exceed the amount of the gift card.

4. After tapping the _Purchase_ button, the Google Play Store will automatically display a credit card payment option, but you can tap the small arrow next to it to change the payment method. Tap the second option to activate your code.