While you cannot buy individual single-trip paper tickets online for home delivery, you can purchase Paris Metro fares digitally through official mobile apps or specialized visitor passes. Physical paper "carnets" (books of 10 tickets) have been phased out in favor of contactless options. Digital Ticketing via Smartphone
If you prefer not to use an app, you can easily buy tickets upon arrival: buy paris metro tickets online
From today, Paris t+ metro/RER/bus tickets no longer exist ... - Facebook While you cannot buy individual single-trip paper tickets