Brownells has come out with their own .308/ 7.62x51mm twenty round magazines for the AR platform. They have the brown anti tilt followers as well. They are retailing for $19.99 from the standard website and $16.97 if you have a dealer/discount account on their website. Other .308 AR magazines also available on Brownells retail for $22.99 (Pro Mag) and $28.99 (DPMS). Either way, a pretty good price for .308 AR magazines and from a reputable maker. From the description on the website-

Designed specifically for DPMS LR-308 and Knight’s SR-25 pattern rifles

Built to same no-compromise quality standards as our AR-15/M16 mags

New-design follower keeps even pressure on the round stack through entire feed path

Straight mag body holds a full 20 rounds – fits existing mag pouches

Plus. . . Rugged heat-treated aluminum construction

Anti-corrosion hard-anodized finish

Dry-film lubricant for smooth insertion in mag well

Stainless steel spring

Removable aluminum floorplate

Unrelated to the .308 magazines, but this is a product video Brownells made about their .223/ 5.56x45mm magazines.