The parts available online have been entered over the years after a hobbyist requests the archive staff to find it and scan it. If you want a part that isn't already online you can use their form and request the part. I'd suggest including the part number, name, approx date, as well as a description of what the item actually is if it isn't easily deduced from the part name. Once they find it they'll scan you a hi-res copy for about $35. They then upload it to their website for anyone to find.
If you want to see if your part is already posted realize that the part number is not entered into a searchable field on their site so you won't find any results searching A-12345. You'll want to search the part name. Use the year filter to narrow the results down.
If you want help let me know as I've done it a lot.
-Tim
