Allied Armaments now has .308 cal 50rd drums available for the HK-91/PTR, FAL and M14 rifles. A bit of web browsing indicates CDNN seems to have the best price. Not cheap at $289.99..........but cheaper than other sellers that I saw.
Here's a link to CDNN's M14 drum mag: http://www.cdnninvestments.com/m130850xdrne.html
and a review of the FAL drum mag: http://commanderzero.com/?p=2530
CommanderZero, a GT poster, did a review earlier of the HK-91/PTR version on his blog.