I voted "other," because none of the available choices quite fits for me.
By quantity of books purchased, my single most common method of eBook discovery is to purchase the monthly WebScriptions bundle from Baen. It's cheap enough, and reliably contains enough books that I like to be a no-brainer. For me, that is; your mileage may vary.
That said, I also have authors I follow. And I frequently check new arrivals at the library. And get recommendations from friends, family, folks hear at MR, etc. And...
None of these methods comes close to being a majority.