Below is the closest thing we have to a definitive list, but you really have to decide for yourself if you want to count a minor league card issued during the year of the MLB debut (e.g., Baltimore News Ruth), a minor league card that was issued before the year of the rookie debut (e.g., Tip Top Lefty Groves), or after the qualifying rookie year (e.g., pretty much anything nominated as a rookie card for Stan Musial), and whether you count postcards (e.g.,
this Walter Johnson postcard), discs (e.g., E254 Tris Speaker), various premia (e.g., M101-1 Honus Wagner), whether the player has to appear alone or can share the card or "card" with another player or two or three...
http://www.net54baseball.com/showthread.php?t=141603