Hello!
If you don't want to use a debit or credit card, the best way will be a Google Play Store Gift Card. These can be purchased at most retailers and gas stations for varying amounts. Here is how you add one to your account.
1. Open the Google Play Store app
2. Tap Menu Redeem
3. Enter your code
4. Tap Redeem
If you have Google Play Store Credit on your account it will show you before purchase that you'd like to select that usable amount before charging another payment type!