Duck Stamp from prior season

I was fooling around and noticed I have duck stamps from 2009 through current except for 2011-2012; I was thinking once I have 10 years worth framing them like you see at DU dinners, but I hate to have a gap, is there anywhere you can buy past years?

2011-12 Duck Stamp

http://www.duckstampdealer.com/federal-duck-stamps

http://ndscs.org/dealer.html