TacticalBarbell.com

I have came across this source. Rowing clubs post their rowers here http://www.row2k.com/ across the country.
Just additional information besides Craiglists if anyone is interested.

http://www.row2k.com/classifieds/classifieds.pl

I'll piggy back off of yours. If you live in the Washington DC area, this event here sells them after they are done.
http://ergsprints.com/buy-a-race-erg/

And this one on Boston area:

http://www.crash-b.org/regatta/ergometer-sales/