What specifically is : "2K Clear" ? Brand , where to buy ?
2K clear is a two part epoxy paint where you either mix the two together for a paint gun, or pierce the inner chamber with a nub on the bottom of a spray can, so once it is mixed together you have limited time to use it.
Given the question this means you'd get the rattle can, and only use what you can within ~48 hours, so it is a bit expensive at ~$25/can unless you have a lot of headlights or other clear coat painting to do at once.
Some people have had luck storing it in the freezer after each use, warming it up before each use, and get 2-3 weeks out of a can before it goes bad (cures in the can).
Brands, most economical would be anything with a lot of good reviews on Amazon.
Personally, for maintenance after refinishing, I just use the same synthetic polishing/cleaner sealant pastes that are meant for body paint, applied every 6 mos. or as needed. There is no need to strip the old off, or mask around the headlight, and same product is great for protecting anything else that doesn't have clear-coated paint... tools, equipment, etc. You don't really want a polishing/cleaner for clear coat unless it's really bad.