Cow hide can be obtained by killing Cows, or by thieving from the Rellekka Leather stall, requiring level 15 Thieving.
Cow hides can be turned into Leather or Hard leather by taking them to a tanner -183 / 64 / 250 and paying a small fee. The tanned hides can be crafted into Leather and Hard leather armor at a crafting table via the Crafting skill.