In US sizes, you generally calculate by adding one size to your mens size. I'm a 9 1/2 mens, so I (optimally) can wear a size 10 1/2 womens. For boots, I've bought size 11 and just worn thick socks. Size 10 can be a bit tight for me I can pull it off with sandles sometimes.
Anyway, your best bet may be to shop online where size selections tend to be greater. I'm sure there are specialty dealers who handle larger sizes, so you'll want to look around. Shoes are one type of clothing I pretty much have to shop for in stores, myself.