Proton.me: “We will be adding Monero as a payment option for our VPN plans by the end of Summer 2025. Source: https://x.com/davidgpeterson/status/1941500514803548636” https://protonmail.uservoice.com/forums/935538-accounts-payments/suggestions/33188971-monero-payment-option
Proton staff says that one can purchase through these authorized proxies:
at least one of these supports Monero.