Where can I get an authentic military backpack?

Well if you are talking U.S. Military, then you can buy one from one of the primary contractors who makes packs for the military.

http://www.mysteryranch.com/

Interestingly enough, both Blackhawk and MysteryRanch have production facilities next to each other in my neck of the woods.

The best place is www.uscav.com They carry everything.

As a former Infantryman, I can personally recommend either Ranger Joe's or Commando Military Supply. They both have a brick-and-mortar locations outside of Fort Benning, Georgia that we would go to frequently. They also have extensive websites.

http://www.rangerjoes.com/

http://www.commandosupply.com

I gotta surplus from my stint with the S.A.F. Doesn't have the metal frame like those Alice packs though.

www.cheaperthandirt.com has pretty good deals too

I've bought backpacks from Maxpedition, Blackhawk and other manufacturers. They were all excellent but not "perfect". I now have IMHO the "perfect" all-around military-style daypack. The Rush 24 by 5.11 Tactical has the Molle set up, multiple pockets, excellent organization and, most importantly, is durable yet the softest of all the cordura nylon packs out there. They also have the Rush 72 for those who want a larger pack. 5.11 also has superior accessory pouches with a proprietary SlickStick system that beats any other attachment system I've come across.

I'm a Navy Veteran and I missed my rugged gear from my service days. You can go on LA POLICE GEAR.
then go to Maxpedition or 511 Tactical. There is a great bag called a bailout bag. A must buy.

I've purchased from Silverman's in the past. They carry a good assortment of patrol packs, etc.

Authentic? You can be real man and join the military rather than trying to play soldier boy dress up.

OTOH, if you simply want a really good, really durable pack then don't get a military issue pack! Most civilian packs from companies such as Osprey, Lowe, Mystery Ranch, etc. are better than what the military issues. When I was in the Army I ditched my ALICE pack (yeah, that's showing my age!) and bought a London Bridge Trading pack.

Try an army/navy surplus store.