I'm not sure that they make 1/2 sizes. I think a 12 would work just fine. Lots of places have free shipping if ordering online. Order a 12 and a 13 and return one. I think I ended up ordering 3 different sizes until I found one that fits.
As others have mentioned, they are a great for sitting around in the tree stand or on the ice. not great if doing a fair amount of walking. I wear regular shoes on the way to the lake and then put them on when I get there (all my lakes are an hour or more away).