Found a neat site called cardpool.com, I usually buy gift cards at a discount there. I'm sure there are others too.
usually they have walmart cards at at 3% discount - its like getting an additional 3% cashback. (on top of your credit card cashback, since you buy the card at this website using your credit card anyway).
there are many other store's cards also available here, but I've mostly bought walmart's since i use that anyway.
PS - not promoting or affiliated with that site in any way, just sharing what i do