If you need a cheap Linux desktop, or a cheap Linux server, there is a ton of x86-based options.
It's when you need all these numerous GPIO pins is when an RPi starts making serious sense. (But an ESP32 could do, too, if running Linux is not a hard requirement.)
I reused the rPis into a k3s cluster for experimentation so they aren't going to waste.
At this point, I think I'm going to rely on esp32 + thin clients for most of my projects. The only uses for rPis that I can still think of is a PiKVM, rPi Weather Station, and maybe a cyberdeck (mostly for the low power usage).
I ended up going with a Wyse 5060, it's a quad-core machine at 2.4ghz + 4gb of ram. It was pretty widely available for ~$35 on ebay so I figured even if it didn't work out, I wouldn't be out too much money.
As far as other retailers go, every discussion I found online basically told me to look at the usual places. eBay, FB marketplace, maybe craigslist. Unfortunately I didn't find any independent retailers.
Also, depending on your use case, you might want to look into AIO thin clients. You can pick up a full machine with a screen for under $50. Unfortunately, the specs on those machines were a little underpowered for what I needed, but it might be a better fit for other projects.
[0]: https://www.parkytowers.me.uk/thin/hware/hardware.shtml
People have adapted all manner of thin, thick and virtual clients to run Klipper. I had it running on a VM in VirtualBox before I moved it to a Jetson Nano.