Still common as dirt around here and priced much the same (since they no longer serve as the default PC/phone controller for many then they got even more available). Trying to find a second hand PS4 controller on the other hand...
You might have a harder time finding one of the fancy rotating dpad controllers, or maybe some of the flight sticks, however the basic wired and wireless options (batteries might be fun) are everywhere for me.
There was a mod once to use USB keyboard and mouse (though today many will point you at the external devices instead which work even on stock), and some of the homebrew will support a few things as well.
As far as "can plug in random USB-HID or direct input compatible controller and go from there" then nothing I am particularly aware of or would want to use in anger,