As it stands, I think the only ways to get grand souls are humans, some bosses at later levels and mammoths. I hope that Dawnguard will add another basic respawning creature with a grand soul. Perhaps in that Soul Cairn place? Who knows, but I'm tired of farming mammoths :/

Well I think the point is, it's SUPPOSED to be harder to get a grand soul. Just like it's harder to find daedric arrows than it steel or other types.

Problem is it's not difficult, I don't value my grand souls any more than I would if they were a little bit more common, it's just tedious to have to farm the same creature to get the soul gems that you actually want to use for your gear.