For 7.62x51 AP, you would have to find some that was put on the market prior to the Reagan ban in '86. It commands pretty high prices due to the fact that none has been put on the market since '86. Or you could do like was previously suggested and get some M2 AP .30-06, pull the bullets, and hand-load them into .308. Doing that, you could increase the velocity a little over the military issue stuff and get even better penetration out of them if you want. Sometimes you can find pulled M2 AP bullets being sold by themselves.
I've given some thought to pulling some of these and loading them for .300 Win Mag to get some serious penetration!
Anyway good luck finding what you are looking for. I would say search gunbroker, armslist, and local gun shows.