From movies, mostly.
The seminal movie in the genre was George Romero’s Night of the Living Dead. Released in 1968, the low budget film made about $30 million worldwide – comparable to about $192 million today – making it among the most commerically successful zombie films of all time.
Dawn of the Dead, released in 1979, also did good business, making about $55 million, or the equivalent of $172 million in inflation adjusted (2012) dollars.
By the way – a movie ticket in 1968 cost about $1.30. So, by this estimate, 23 million people watched Night of the Living Dead in theaters. It then had a very long run as a “midnight movie” and revival house staple, plus a multi-decade run as a popular home video rental. The same would also have been true for Dawn of the Dead. In 1979, when a movie ticket was about $2.50, perhaps 22 million people saw the movie in its initial theatrical release, and then countless more saw it in revival showings and on video.
These films were commerically successful and they defined the genre. Zombies movies incorporate the basics of the genre as defined by the early Romero movies – zombies are reanimated dead, feasting on human flesh, that can only be killed with a head-shot. Secondary themes that generally appear include: the end of society as we know it; and, generally speaking, other survivors are as big a threat as the zombies themselves.
The various zombie movies that come after – Romero’s subsequent films, zombie comedies like Shaun of the Dead and Zombieland, the remakes of the Romero films (including the kind-of great 2004 Dawn of the Dead) – stick with the rules even if they don’t embrace every theme.
The real success in the zombie-movie arena is the Resident Evil series, based on a video game. The first Resident Evil movie (2002) made about $129 million (inflation-adjusted), and the last two – Afterlife and Retribution – are the highest grossing zombie movies of all time (with about 80% of the receipts coming from foreign ticket sales).
That assumes, though, that you consider the Resident Evil series to be zombie movies, because maybe they aren’t.
First, they are exemplars of a totally different genre – the video game adaptation (they are in fact the most successful video game adaption movies ever made). Watching them you do feel like you are watching a video game. The action is entirely linear. The protagonist group moves from setting to setting accomplishing tasks, fighting an endless stream of opponents (some zombies, some other stuff – like various forms of mutants and super-monsters), and managing inventory (bullets, weapons, tools) in order to move to the next level (to use the video game language).
Second, they don’t obey some basic zombie conventions. For example, head-shots to the brain are not required to kill the zombie-like creatures.
So, one might exclude these movies from the genre because they fail to engage with the genre in a serious fashion. Zombies are background noise in these movies; the foreground is taken up with martial arts acrobatics, Matrix-like special effects, and first-person shooter action. While all zombie movies are about other things as well, these movies aren’t really about zombies at all.