Many, many years ago. I particularly remember hunting down front brake pads for the Fiat Dino 2400 Coupe that I used to have before I bought the Porto 20vt, I could easily wear out a new set after one track day round Goodwood or Castle Combe. I also used to look for suspension and brake parts for the old 124 Spider. I guess you would want to look for service parts and other mechanical and electrical parts that are getting hard to find, cooling system (16v thermostats for example, radiators maybe) brake parts (seal kits, etc.) ..? Plus any parts you might want to replace tired or broken parts, eg: headlights, bonnet release levers, etc.
Armed with a list of elusive parts, I'd then check OE part numbers on ePer (both original and any replacements) and all aftermarket equivalents.
Also look out for brochures and press kits, if you don't already have a full set
And just in case, take some large bags, and cash to negotiate best prices (maybe even fill the bags with cash to start? (although these days maybe contactless is king, not cash).
Plus an umbrella for Sunday. [Edit: I thought it was this weekend but it's next weekend]
And whatever you do, avoid the cars for sale areas...