Reseach, research, research on the internet
I'm well aware "big foot" as I once worked outdoor goods and am very well aware of the lack of numerous products for the small and the big.
I'm in the cusp, just within the sizes usually distributed retail. know first hand how lucky I was.
I'd start by keying in such as "size 16 insulated boot" etc., and work from there.
At least, now there is an internet, instead of attempting to buy just locally, or by traveling.