I purchased these on eBay just last month and I'm keeping one, selling the other.
http://www.ebay.com/itm/2-Autographe...c#ht_403wt_917
Please read the auction description carefully. There is no way to prove if these acutally came from Clemens' 20K game or not. Only the auto is certified. I believe they are game used, but I don't believe each ball was used for that particular strikeout. I believe he just signed 20 random game used balls after the game and numbered them 1-20 to represent 20 K's. I do know they are authentic Roger Clemens autos early from his career, which there aren't that many of because he didn't sign a lot of stuff early in his career and his signature has changed over the years.
I'm keeping the one on the right, selling the one on the left. I'm bascially selling it for what I have into it. $80 delivered. Any questions, please ask.