How much money are you looking to spend?
Federal, Remington, and Winchester produce some quality brass cased non-corrosive ammo that shoot decent groups through a Vepr K in 7.62x39 - Be prepared to pay $6+ per box of 20 cartridges though.
However, I found Barnaul 125gr FMJ to shoot very tight groups and to be the best overall 7.62x39
Of course its steel cased and AR owners are rather allergic to using steel cased ammo in their AR.
I have an AR-15 that I built, and a Vepr K. The Vepr chews up whatever ammo I use, but for the AR I stick to proven brass-cased ammo just because it was designed for it. The Veprs and AKs were designed for steel cased ammo. That being said, I think the worst you can do to your AR 7.62x39 upper shooting steel cased ammo is wear out the extractor. They're not all that expensive and easy to replace.
a gun shop that i buy alot from has some south african 7.62x39 in clear heavy plastic battle packs. If you need a price, call 228-475-4700. Guns and Ammo is the name of the store.