If you click on the eBay search link above then 'Save this search', eBay will e-mail you as soon as 3rd row seats come up for sale.
I used this feature to find mine. In the 6 months I was observing the eBay market (August 2006 to January '07) at least 6 pairs were sold. (3 grey, 2 tan/java, 1 black/anthracite.)
There was a brand new pair of anthracite seats that have been offered several times but didn't sell as they were overpriced. A black pair did sell in October for US$1425, but that was seats only - no seatbelts or brackets. Black seats tend to attract higher prices at auction as they are less common.