The vast majority of them are gathered from various pre-existing collections (https://cyber.dabamos.de/88x31/ and https://anlucas.neocities.org/88x31Buttons are the main ones, with some smaller personal website collections i came across)
But some of the rarer ones are found by scouting, often ending up on old japanese websites that are still running (i remember one time some sort of a car webring that linked to a lot of other sites? very vague memory from a year or two ago)
Were am i gonna put the css joy webring