"itsmyforte" is absolutely correct. The matching center speaker for the KSB-2.1 is the KSC-C1. Last time I looked on ebay, (20 minutes ago) there were no KSC models for sale. But, don't give up. Keep looking. I just saw two pair of KSB-S1 surrounds sell in the last couple of days that are the "perfect match" surrounds for you KSB-2.1's.
By the way, where did you find new KSB-2.1 speakers? They have been out of production for a couple of years now. Maybe you have a source for older model Klipsch speakers that many folks would be interested in. Please advise.
Finally, I have two pair of "perfectly matched" KSB-1.1's that I am about to replace. Let me know if you need any more of the KSB line (original boxes, paperwork, etc.)