Yes and no. There are loads of shoe shops in town, most of which sells wellies. Most souvenir places won't per se, although neon-colored robust plastic bags which come with ties for securing around ones knees are becoming more common and are the product most marketed for the tourists among foot-protectors. Wellies are really cheap here though because they're so necessary and so widely available. Still, most acqua alta isn't really all that high and can be avoided. S. Marco is the lowest part of the city though, so if it's going to flood anywhere, it'll flood there.