Soy milk is quite common in Japan, you can find them easily in most supermarkets and many convenience stores. There are two types, real soy-taste soy milk (you can make tofu from some of them) and those adjusted and flavoured.
Recently some supermarkets started selling almond milk as well, import from the US I believe. It is still rather rare, not so common nor well-known yet.
Soy milk is usually sold in cartons, 1 litre or 250ml. Prices are similar to milk, or slightly higher, it depends on the brands and types.
Soy milk is called 豆乳(とうにゅう) tōnyū, meaning "bean milk".