I've never seen a good one. I saw a laughably bad fake Jack Johnson 'signed' one once.
Boxing autographed cards are tougher than comparable baseball cards. I guess it wasn't that commonplace back in the day to get boxers to sign. Most of the ones I've seen [prewar] are cabinet cards or postcards, with a smattering of exhibits. I have a nice assortment of signed exhibit boxing cards displayed on my web site:
http://imageevent.com/exhibitman/aut...edexhibitcards
Postwar there are more to be found. I've seen nice 1948 Leaf and 1951 Ringside signed collections [once upon a time I owned a really nice group that I sold off; still regret that], and more exhibit cards. The really modern stuff is easy, of course, but even then there are some relative rarities, mostly the guys who are on the other side of the world, like South African HOFer Brian Mitchell, and who can't readily do TTM signings, or guys who just don't like signing cards, like Nigel Benn.