Until recently people were buying it from India to treat FIV (a previously fatal disease in housecats). There was a whole network of people supplying it through Facebook groups and mailing it to the US.
To fill in slightly more detail, that's GS-441524 instead of remdesivir. It's closely related but a lot easier to manufacture. Here's a good article on that:
https://www.statnews.com/2020/05/14/gilead-should-ditch-remd...
And a fascinating tale how cat owners are buying the black market version to treat FIP:
https://www.theatlantic.com/science/archive/2020/05/remdesiv...