There certainly is such a thing as pork stock, it's made the same way as beef and chicken stock - by cooking down bones to extract the flavors. With beef you use beef bones, with pork you use pork bones.
Supermarkets stock the products that make economic sense, they don't carry items which won't move - especially if they have a very limited shelf life. Beef, chicken and vegetable stocks are the most widely used ones in many parts of the world, so they will carry those as they have a good chance to be sold. Pork, lamb, and fish stocks can be found in supermarkets in areas where enough of the customer base will buy it.
If you can't find it in your local supermarket then there may be specialty stores (online or bricks and mortar) which you could get it from. Butchers may carry it as well. If not then making stock is very simple, all you need is some bones, vegetables, a heat source and some time.