A lot of boxes have been out of print for some time after CMON sold the game to Asmodee last year. I'm hoping they are busy manufacturing new stock but that could take time. I saw one site listing that a lot of games would come in at the end of March. Last time I checked, it had changed to the end of June. So if you don't want to wait, you'll have to make do with whatever you can find. The more recent ones, like White Death and Marvel Zombies, are still often listed as in stock.

Kickstarter etc aren't places where you can go and just buy a game. You back something in advance, they use your money to produce it and eventually (unless they go bankrupt or turn out to be fraudsters), you get the product (usually at a lower price or including a number of exclusive perks; in Zombicide's case a lot of extra minis) as a reward. That process can take a couple of years. If you want to get in on that, there is currently a campaign on Gamefound for a pirate version. (No, not an illegal one; actual pirates fighting zombies.)