You either need a lab to test donor samples first, or when this was more of a craze, a popular source of 'donations' for the DIY crowd was young children.

If you're interested in digging into the people that were doing this, they had a website dedicated to everyone telling their stories of how they went about their own individual journeys.

The website was called thepowerofpoop.com and looks like it's gone now, but is available on the wayback machine including individual articles and images.

I would go back to at least 2022 .. I think they possibly got in legal trouble at some point and started taking things down.