Most people who buy it get it from steam, you may be able to find it at some game retailer but probably not as easily (Like gamestop)
Pirating it is also an option but... y'know that illegal and you're kind of robbing rockstar who deserves it for the great game
When you want to start installing mods, head over to the "Mod Releases" section and download something. Usually they will have instructions somewhere in the topic or readme.txt. There is this one mod called vehicle spawner by MadmaN and it got a video tutorial created for how to install it by Red Blaster. If you watch the video I bet it will help as well, even if you don't want that specific mod. My Super Mod 3 also has a pretty good readme.txt file which is pretty easy to understand.
