What Mary has quoted there is essentially correct ... Meet & Greet tickets are only available through the PBS station which is sponsoring the concert you are attending. They are primarily offered during Pledge drives and are technically not "sold" but are offered as a "Thank You Gift" in return for a pledge to that particular station. The Pledge size varies depending on the station and what all is included.
Some stations will only allow these to be obtained during their pledge shows, while other stations will allow you to contact the station directly, both before and after the pledge show airs. Since there are usually no more than 20-40 M&G spots available, they usually go very fast.
Sometimes you can find someone who has an extra M&G pass available here on the forums at this link: http://www.celticwomanforum.com/index.php?board=39.0
This is usually because these are often sold as pairs, so if someone pledges for a pair, but is actually attending solo, they may offer the second ticket and/or pass for someone who's interested.