The casting resins aren't as hard as horn. I've used most casting resins and whilst they are hard enough to polish they aren't as hard/durable as buffalo horn.
Definitely get your horn from an archery supplier.
Although I haven't submitted them to a hardness test, for the poundages that I'm used to working in (less than 60#) there's no problems using the casting resin. Then again, I've never been one for high polish looks of anything... The bows that I make get beat up by dozens of people every year who have no clue which end of a bow to point towards a target, so mine are usually... Ummm... "Utilitarian" in construction.
If you want a meat-maker or bulls-eye puncher as opposed to a dust-collecting wall hanger, then a lack of a high luster polish is more than made up for in ease of obtainability. But, that's just me. Cheers!