Hk mags are at an all time low right now. I think they're even less expensive than in the early 80's.
Check out Tapco, InterOrdnance, Federal Arms, and a few others in the Shotgun News.
I've found Ebay to be a great source for inexpensive stuff. You must have the discipline to avoid bidding above your original intentions. This often takes patience, but I've saved around $500 over buying similar stuff from a shop (not necessarily talking about gun related items here).