This site's hanging pots only go up to 12".
But they do sell wire hangers so you could probably hang something bigger.
http://plasticflowerpots.net/w...
Griowers' Solutions hanging baskets only go up to 12"
http://www.growerssolution.com...
Greenhouse Megastore : I looked at several of these and only saw 12" and smaller.
http://www.greenhousemegastore...
Even their hangers only advertise up to 12"!
http://www.greenhousemegastore...
I wonder if a 16" pot slipped and fell on someone who sued someone, and now no one will sell them?
Maybe a botanical garden could tell you where they buy theirs. Or a local extension service or Master Gardeners' phone line would know someone with huge hanges, and THEY could suggest a source.
I'm starting to think that you may have to make your own. Make it strong!