If you cannot find good boots intended for paddling and can use a general waterproof boot, consider a pair of “Alaska boots”. They are not insulated, but because they are used by all kinds of boaters, they come in a huge size range.
I bought a pair and was amazed that they had ones for very small feet (kids, small women) as well as for, well, clodhoppers. The ones I bought had somewhat stiff vinyl-like material, thick tough soles, and a snug fit around the ankles (NOT floppy). They cost something like $10 back in 2004…in Alaska. I think every resident must’ve owned at least one pair.
They will let more water flood in than Chota Mukluks if you get in water higher than the tops, but they also dry much faster.
Whether they’ll work for you depends partly on the kayak’s fit at the feet.
The Scuba shop (first link) sells several models of Neosport neoprene socks in XXL which they list as fitting sizes 14-15 as well as dive boots in individual sizes up to size 16. I have a pair of similar Deep See brand neoprene short dive boots that I often wear kayaking or canoeing. They also have thinner neoprene socks that you could wear with water sandals (like the Keens below).
This other US company (second link) has 373 styles of shoes in size 16, even up to size 18 (actually, US 15.5 is equal to UK 16 or Euro 50 but close enough). They have the Keen closed toe sandals that a lot of folks wear for paddling, often with neoprene socks. My late uncle was 6’ 7" and wore size 15 shoes and I remembered he ordered all his shoes from these folks.
I just ordered a Palm drysuit from this company, Watersports Outlet, which is based in the UK but has USA outlets in Michigan and California. They have one pair of size 15 US lace up paddling boots on clearance sale but they seem to be in the US, not listed when you switch the site catalog to UK. Usually these companies can provide you with measurements that might help you determine if these might fit: