You can buy at several places that interop with each other—iTunes and Amazon are the two biggest. They don’t have literally every movie, but they have most that most people want to watch. https://moviesanywhere.com/participants

Cost ranges from $5-30. Fewer dirt cheap sales than Steam, but the standard price point at launch is lower, in exchange.

(Having to explain “buying movies” makes me feel old!)

Unlike with steam, things can appear disappear from your iTunes library if you move countries. At least music can.