As far as domestic cattle goes, BULL horns have more solid material near the tip than COW(female) horns do. Ask your local butchers to save any bull horns for you. Most will.
If you don't want to wait, you can buy horn on the big auction site. I have bought water buffalo horn in both solid 'slabs' about 3/4" thick, and 'tips'. The tips work better for English longbow types. Do a search there for "buffalo solid rough horn".