Recently I got the (strong) itch to upgrade my panel to a 12" 1024x768 variant, and was lucky to find one on Amazon. It was on the expensive side, but I tore into it regardless, and was able to mount it successfully within my Picade. I’ve uploaded an album to Imgur which has image descriptions that explains the story, so I’ll keep this message here brief.
I hope this helps anyone still looking for a 4:3 small LCD panel, as it does seem they’re becoming quite tough to obtain indeed! These Eyoyo ones do seem to be sprinkled about though, so might be a viable option.
Cheers, and thanks to the good ship Pimoroni for creating such a wonderful kit!