If you cook just about any piece of meat that has a joint of some kind, you will get collagen in the jelly and juices.
I quite often cook a piece of pork neck in a cast iron pot. The juices in the bottom separate out as they cool into a layer of jelly, a layer of thick meat juice and on top a layer of fat. It's quite easy to scrape the fat off the top (I use it for frying mushrooms and onions), team off the juices for gravy and sauce, and collect the jelly for sauces or just for spreading on bread.
Different parts of the carcases of different animals will give difference proportions of jelly, juices and fat, but in general anything with more than one bone will give you a lot. Shoulder, neck, hocks, knees, all are good. The bigger the animal, the bigger the bones; and the bigger the bones, the more connective tissue there is, and therefore the more jelly you'll get.
You'll even get some jelly from ducks and chickens, but less than from cows, pigs and sheep.
If you go to a proper butcher who gets in whole or half carcases to cut into joints, then the bones will still be in the "for human consumption" circuit. My butchers will ask me if I want bones when I buy cuts like beef shin, and give them to me for free (but this is in France, it's been many years since I bought stewing beef in the UK).