Are you certain the card is a genuine branded one? The vast majority of memory card issues are due to fake cards, mostly sold by online retailers. Sad to say, it's almost hard to avoid them these days.
The first step I would suggest is to check yours against the points noted on this page, or get hold of a known genuine card and see if it makes a difference.
If you stick to major retailers you should be ok. There is quite a lot of variance in price even amonst the big stores, but in general it's the usual 'buyer beware' adage: if the price looks too good to be true, it probably is.
I'd avoid all auction sites (you know the ones I mean) regardless of putative reputation of sellers, and while I can recommend Amazon.co.uk (good prices and they've never let me down) I am wary of using Amazon Marketplace for the same reasons.
Another worry-free route would be manufacturers' websites, which often have 'specials' and very resonable pricing.