need some info on some old memorabilia.?

when i was a kid we used to write to football and baseball teams, and they would send back pics, stickers, and all kind of stuff. well i was lookin thru them and i found(in the original postmarked envelope 1971) three hand signed black and white cards. one was willie stargell, the other manny sangienne and last but not least, roberto clemente, all mint, perfect. any place to get a value? pls let me know tnx.