Costco doesn't sell anything Apple any more. You can still get discounted cards at BJ's or Sams Club. CVS sometimes has ExtraBucks promos with cards and Target sometimes puts them on sale.
I have seen them at Sams. You can get them at Target with their RedCard that will give you 5% off. Sometimes they offer them at a discount and you get the 5% off with the RedCard.
The best deals are usually around Christmas time. Safeway/Randalls/Tom Thumb ran a special the past two years were I got $60 iTunes gift cards for $34.