Should appear when you search "Riot points"
But here you go- https://www.amazon.com/League-Legends-25-Gift-Card/dp/B0153XBEBM/ref=sr_1_1?keywords=Riot%2Bpoints&qid=1577348179&sr=8-1&th=1
They are.
https://www.amazon.com/League-Legends-25-Gift-Card/dp/B0153XBEBM/ref=sr_1_1?s=videogames&ie=UTF8&qid=1466373630&sr=1-1&keywords=riot+card