I have the three-disc Criterion Collection release of Hard Boiled, which is fantastic, except that side 5 has mild rot. It's not unwatchable but it irks me.
Rather than trying to track down an entire replacement set, is there anywhere where one might be able to try and trade for / buy just one disc from a set?

If you are lucky and someone has the extra disc that reads the forum then you can get it.
But your only real option is to buy another set to make 1 good set for yourself.

Good thing is that you have one and if you get another one with worse sleeve you can get it cheaper, or better sleeve etc.