Are there mid mount mowers available for older compacts either Deere or ideally third party ? Or is it strictly about finding a specific old mower (which is very rarely separated).
Check with your dealer. Deere makes a limited number of decks and has unique mounting hardware for each series of tractor. The 3320 isn't that old so the hardware may be available. I doubt there would be any third party support.
It’s possible with enough searching. I just added a mid mounted mower to my 4100. Found the mower on marketplace with most of the linkage, sourced the missing parts from eBay and Deere. Considering my tractor is 23 years old I’d say you have a better shot of finding everything to make it work.