Instructions for how to turn any gif (or meme) into a Click to Show are in the comments of this meme:
imgflip.com/i/vk8n9
It's basically just copying the Embed Codes, pasting it into a comment, then deleting some extra stuff from the front and back.
But as for where they've been sourcing the gifs from, I don't know. I guess if you see someone put a Click to Show that want to use, you can use the number and letter code to go to the original gif (by typing that after "imgflip.com/gif/" at the top of the browser), and then get the Embed Codes from there.
Let me know if that helps.
(Or otherwise just ask Craziness_all_the_way, DashHopes etc. where they're getting them.)