The problem with cable and satellite companies offering packages is that it’s provided from the broadcasting companies in that way. “Want ESPN? You have to put 2 channels of disney in the same tier” sort of agreements. The FCC has considered a la carte TV in the past, but it would require regulation in places in the TV food chain you’d never even think of.
So just getting Discovery is probably impossible though cable or satellite providers, unless you can find it unencrypted on a satellite downlink and pick it up with a large dish, but I’m pretty darn sure that it’s encrypted—meaning you’d have to buy a receiver from Discovery Networks.
If you are just looking for something to watch in that way, Lyngsat keeps a free tv list updated. You can pull these down with a free to air receiver.