Can you buy AR coating?

In a bottle or a spray to apply to glass?

I’m afraid not.
They are traditionally applied in a vacuum. Some methods may not require a vacuum anymore but you will still need special equipment.
rp-photonics.com dielectric coatings There is a fabrication section.

why you need that anyway?
have any plan to coated other than flashlight lenses?

windows, car windshields, beakers, drinking glasses, light bulbs, cell phone, or anything else made of glass? :stuck_out_tongue: