Unless the cow is a "Meadow Ornament" you are not going to see a lot of domestic horns anymore. As was said, they are dehorned to safeguard the cows from each other.
IN 2001, we had the great hoof in mouth problem that caused many, many cows to be slaughtered and burned/buried. Such a shame.
As Rick mentioned, most all horns available now in commercial quantities are from Mexico, South America, Africa or India. Of these, the South American ones are the best.
In these days of constant unrest, it has become more and more difficult to even import horns in quantity. The USDA which has to inspect and approve the shipment is now under the Department of Homeland Security. And those people are next to impossible to deal with. Anyone who is importing quantities of horns (and I only really know of two or three) have to be either very dedicated or stone crazy. Been there, done that, and I probably qualify on both accounts.
John