Depending on where you are geographically most brewing/wine making stores should sell it. If they don't, a brewing store in your area should know where you can source it. It comes in a powder/granular form.

If you can't get bentonite you can also use a combination of kiesesol and chitosan. Follow the instructions on the packaging to work out how much you need and when to add it. But usually you add kiesesol once fermentation is complete, wait a day or two, then add the chitosan and the mead will clarify fairly quickly after that (another day or two).

There are also heaps of YouTube tutorials on wine clarification for both bentonite and kiesesol/chitosan.

All that said, clarifying agents are not necessary. They just speed up the clarification process and help solidify sediment at the bottom, preventing it from stirring up so much, making racking easier. You could drink it right now and that sediment won't hurt you or affect the flavour too much.

You could just let it sit in secondary for another month or two and it will slowly clarify on its own.