I'm going to go ahead and take the liberty of assuming that you perhaps misused the work "authentic". When you say authentic, I wonder if you mean "professional", "not crappy", flashlights. There are flashlights at home depot, ebay and your local hardware store that are barely worthy of the word flashlight. I suppose it depends on your definition of authentisity, but good quality flashlights are easy to find:
Some good brands are:
- Surefire; not to be confused with anything else ending in ****fire. (eg, Uberfire, Megafire, Coolfire, Superfire are all knockoffs).
- Fenix
- Sunwayman
- Zebralight
- 4Sevens (The Quark series; I love the word "Quark". Quark quark quark...)
- Nitecore
- Jetbeam
- Eagletac
Any of these brands will serve you well. There are others, as you will see from scrolling down the forums.
There are many reputable dealers. Often the manufacturers will sell directly. There are outfitter stores like goinggear which have a good selection. I'm in Canada, so I really only know of the Canadian dealers. You should check out CPFmarketplace for more places to buy. They are a separate "sister" forum, exclusively dedicated to flashlight dealing.
What you want to avoid are the no-name brands that you see on ebay. If it has multiple LEDs or has no affiliated name, these are things you want to avoid. Ultimately, knockoff brands are not a problem unless you're buying out of the back of someones van. If you're really worried about it, order from the manufacturer directly.
Hope that helps,
-PW