eBay, Facebook Marketplace, etc. Find one that's at least a few years old with at least a decent CPU and 8GB of RAM (or 4GB if you decide to run Linux). Finding one with issues (i.e. no disk drive, no RAM, etc) is a way to save a bit if you don't mind fixing it up yourself and can get the replacement part(s) for relatively cheap.
Key words like "no os" "no hdd" "no ram" on eBay will get you listings from recyclers and whatnot, which will have lower prices and avoid overpriced refurb listings.
Example: a couple years ago I found an Acer Nitro from 2018 on Facebook Marketplace for $50. Needed a charger and a new disk drive. Was able to get both of those for another $50 or so total. $100 got me a perfectly usable laptop that was only 5-6 years old. It even had a dedicated GPU, good for speeding up video encoding at least a little bit.
For something real basic but a bit on the old side, I can say the Inspiron 5570 with the 8th gen i5 is a good laptop and is pretty cheap now. It was my daily driver until I blew out the audio chip. Now is my web server.