I don't make any of the pixels except the flags. If you want some I haven't posted, check out
http://emojibank.com/
https://gifcities.org/
Tumblr, DeviantArt, Neocities... etc.

If you really wanna make one, pixels are usually 20px x 20px, and here's a tutorial on how to make stamps https://www.deviantart.com/clefairykid/journal/Stamps-For-Beginners-and-Beyond-216751009