Making gelatin from scratch
It's the refinement that's the real issue: anyone who's braised a big joint of meat knows that a couple of hours of low-temperature stewing will net you large amounts of gelatin. Hooves and antlers were the preferred media, but anything that's got a bunch of collagen will work. Talk to your butchers shop, and see if you can buy some bones.

Refinement was done the old fashioned way: by hand. You skim the top to remove the scum, you add egg whites to degrease and to clarify the "broth", and then you strain and strain and strain and strain.

And strain. And strain. And strain. The skimming/straining process is similar to rendering sugar from sugar cane, if you've ever done it (slightly less esoteric, because people still like molasses). They obviously don't use egg whites in rendering sugar.

It's a huge amount of work (and OMG it stinks...do it outdoors if possible), and the end result is probably not going to be on par with the stuff you buy from the store. Unless you're hankering to re-invent an ancient technology just for the fun of it, I'd just buy it.

Edit: I dug around, and there are a number of products for straining jelly that might make it easier, and they should be able to deal with the viscosity of the gelatin soup. Unfortunately they seem to be made for smaller batches, and that may not be helpful if you're doing a huge kettle.