ebuyer's not bad for flash media pricing - most of the flash media generally comes with free delivery (although if you order it along with someone that doesn't have free delivery the system will add its weight to the delivery calculation, so order it separately).
It doesn't matter too much which format you pick - it's more useful to look at the data transfer speeds that the cards you buy offer. The slowest ones around and the fastest ones are an order of magnitude different in speed, and that can mean a picture taking an extra second or two to write to the card, depending on the actual speeds.
In general, though, the majority of cards around offer are sufficiently fast that their speed isn't a particular problem - if you want to write uncompressed files on a high-end SLR, or want to record VGA-resolution 30 fps video, then you may need a specifically high-speed card, but for most digital cameras, price will likely be the most important deciding factor.