You can get great deals on large size shoes at Amazon with a bit of patience. Their own Amazon Essentials range does fashionable styles at great prices up to size 15 US. The women's shoes department goes right up to 16 US, although many of the largest sizes will either be Chinese brands or unisex. If you have Amazon Prime, many shoes are eligible for free, fast shipping. Plus you can even try before you buy.
Custom boots handmade by Miguel in Mexico up to size 16 us. Not just cowboy boots: all sorts of styles including platform, long boots & men's high heels. Choose from a wide range of styles or design your own boots. Free US shipping & ships worldwide. As well as the Etsy store & reviews, you can see more custom boot designs on Instagram.
Specialist large size shoe store in Quebec also selling online to Canada ($10 flat rate shipping) with a wide range of women's shoes in sizes 10 to 15 US, with selected fantasy styles up to size 17 US. Large selection of brands and styles. They even do rain boots, slippers and socks. They are happy to advise on the right size for each style if you send them your foot measurements before ordering or during checkout. You can also visit their shop in Rue St-Hubert, Montreal. Website is in french.