Hi Briony
Depending on your budget and the Speed you require, try looking for the Transcend class 6 SDHC (secure digital high capcity) there range is budget but very good.
And concider would you be better with 2 or 3 low capacity say 1GB or 2GB then your only looking as SD not SDHC and would save a few quid, try Play.com or Amazon there usually pretty good.
Anything else you need feel free to PM
N.