Trusted way:
1. Go to ----> https://www.gyft.com/buy-gift-cards/ then  https://www.gyft.com/buy-gift-cards/google-play/
you can buy cards from 10$ to 100$ and pay using bitcoin.

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2. you can try online sellers like paxful.com https://paxful.com/buy-bitcoin/google-play-gift-card
Can be scammed even with the use of escrow service

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3. use https://spendabit.co/go?q=google+play "I did not try this site nor recommend it"

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4. use this forum and remember to use escrow service