I don't think this is what your looking for, but, Bob at Plainfield shooting Supplys just ordered me some mag extensions and he had them in two days. He also saved me $.50 cents apiece, let alone shipping! I would try there or Braddis. Hope this helped.
Give Russ or Jason or George a call down at Elmores. Last time I was in there they had a few boxes of the 5.56 ones. They may have some .308 ones in as they have a bunch of .308 ARs in stock.