I also wear the dreaded size 16.
The Sorel boots tend to be very wide across the sole which doesn't fit the running boards/foot wells very well (picture getting stuck, or rolling off). They are very warm, but short and fluffy at the top which can let snow it. They don't have a lot of support since they are mostly insulation and can't be snugged up very well.
I bought a pair of insulated leather (11" tops &waterproof) boots from Cabela's I only have one season on them and enjoy their fit and support. These are in a size 16 and are not too wide so they fit the boards well and snug up well to provide support and keep snow out of the top. I will probably replace these when the time comes with the same boot as I am not sure of anything better for the money at this time.
I had looked at HMK ( I think) that had a 16 but they were super narrow. This tends to put the foot in a bind or force it into the toe box. Sore toes are no fun.
As for regular shoes, try
www.oddball.com . These guys are based in Portland Oregon and have shoes (dress, casual, sport, running, hiking, work, etc.) starting at size 12 and go up to somewhere in the 20's. They also specialize in super extreme widths as well. I'm almost convinced you could get square shoes from these guys. It is handy for me to stop in the store and try on anything from a 15 to a 17 to get an exact fit for my foot depending on shoe make and model. They do a tremendous amount of their business by mail order so you don't have to be local to benefit.
I imagine there are other retailers that can help you, but the Cabela's boots and Oddball shoes work for me.
VB