What is the most credible sight to get slender?

So basically, I don't know a lot about the computer game but I have app version, and it seems pretty fun. I want to get the computer game, but I have absolutely no idea where to start. Basically, it seems like a pretty popular game and I would be kind of surprised if it didn't cost any money because, well its really popular. And most of the sights I've found to get it seems kind of shady, and I really don't want to end up getting a virus. So, bottom line, where's the best place to download slender?