Usually end up buying games during Steam sales, or sometimes via Humble Bundle… though nowadays Humble Bundle hasn’t been as interesting after they were bought out and the founders stepped down. Before all that Humble Bundle used to have some great Indie bundles with lots of Linux and Android games.

I would also probably buy from gog.com and itch.io but haven’t really needed to yet.

I’m usually looking around for games with Linux compatibility so any game storefront that ignores Linux gets ignored by me.