Right now the only way to purchase Ripl Pro is through your Apple ID or Google Play account. One option is to subscribe to the annual renewal and expense the charge once a year. Alternatively, you can change the credit card on file for your account for that one purchase. To update your payment method on iTunes, see this article. To update your payment method on Google, see this article.
Alternatively, you can purchase iTunes or Google Play gift cards and use those to purchase the Ripl Pro subscription.