The best place is local farms, slaughter houses and cattle vets. They tend to have bags and bags of them lying around.
If none are local, eBay is often a good option. I've had quite a few really nice cow horn tips from eBay, for a couple of pounds each. You've also got medieval reenactment websites - I've no idea about them in the states but over here they do both sheets of cow horn for arrow inserts and complete horns for various uses.
