I would try buying them on discounts in stores. If you have a loyalty card with a store which allows you to exchange points for a discount that may be a way to go. I regularly exchange my nectar points for itunes vouchers. I would recommend to stay away from places such as ebay, I once purchased a itunes gift card which when I tried to activate gave me an error message that it was un-activated. Apple support told me it was likely that it could have been stolen. Luckily that time I managed to get my money back, but you want to be careful.