Where do you find a mammoths tusk in skyrim?

You can get a mammoth's tusk by killing a mammoth or you can buy one from Lucan, the Riverwood Trader. If you want to get a mammoth's tusk without killing a mammoth or spending gold, you can find a few mammoth's tusks and snouts as well in Halted Stream Camp, but it in that location there will be a couple of bandits that you will have to kill.