I guess anywhere they had Boxing Day specials? Someone has to have SD cards on special. Oh, and I'd think about buying from an online store, but not an eBay type store. At least try a place that looks legit, like B&H Photo-Video. They wouldn't have the cheapest prices though.
And this is definitely the wrong forum for this sort of thing. People come here to chillax. I think you'd get more responses if this thread were moved to the "Buying Tips and Advice" forum. Hopefully a mod will.