Many are actually still available from Deere; I ordered a pumpkin plate from the dealership a few years ago. Plan on around $30 per plate if I'm remembering right. I've also picked up quite a few for a buck each at used equipment yards in areas where vegetables are/were grown. Note that for many of the vegetable plates the correct filler must also be used.
I have a pdf copy of the PC0268 seed plate catalog from 1981 that lists everything they made. Shoot me an email at nafziger (at) mtelco (dot) net and I will send you a copy. This catalog also spells out what filler plates or hopper bottoms are required.