This is on sale on Thursday 18th December. Pack of two (2) x 2 gigabyte SD cards for only £8.76. You might be able to get them cheaper online, but these should be good for homebrew on the Wii, and if you need them before Christmas then these seem like a good shop price. I'm going for them for my family's Wii.
Site says: Traveller high speed SD memory card. No other info at the mo.