I found Dura Ace clips (not sure they're AX, for 7400?) and 105 clips all from the "aero platform era" will mount on my Shimano 6207 600EX pedals. I'm normally not a huge fan of Shimano even of this era, but those pedals are fantastic. I also use the Campy Strada C-record and Chorus in similar styling. I like the C-record best of all, but the proprietary toeclip, more rare because it's C-record, is hard to find.

Depending on the pedal, probably the best way to score replacements is to find a used complete pedal set that needs bearing cups or proprietary cones or new spindles. Likely they will have cosmetic issues, but you will have the vintage riding experience with the correct clip. Also keep in mind Shimano and Campy were selling clips in different lengths. I have no idea if the sizings were compatible across brands. You could end up with clips that do not position your foot as intended.

Another thing is, many people think it is required to use hard cleats and tight straps with such pedals. If you want the full competition experience, of course you can do this. But you do have to train yourself to the habit of extracting your foot early when stopping, which may require reaching down and loosening the straps.

You can ride these pedals in sneakers or street shoes with the straps just barely tight enough to feel. If you do that your feet will come out when you need them. Folks say they will come out just any old time, but that is not my experience.