Yeah, 'bout a week back I went to rent GOWIII at Blockbuster, and was planning on getting $20 card. Guy only had $50 cards left. He said he got a notice from Sony that they weren't going to get any cards until a couple of weeks into September. Something about people buying up the $20 cards for the new MW2 map packs.
I went to Wal-mart after to get Heavy Rain for $30, and they only had $50 cards there aswell.
Anyways, I found a bunch at my nearest 7 Eleven. That's probably your best bet I'd say.