Nintendo doesn't sell any external memory cards, but there are a lot of common SD Cards that the Wii can use for extra memory. "SanDisk" SD Memory Cards are known to be pretty good, and can have up to 2 GB of memory. Just make sure any memory cards you do buy are normal SD. Not "mini, SDHC", or any of those other types of SD cards.
But unless you intend to buy a huge amount of Virtual Console games (a huge number like fifty), you probably won't need any memory cards. 512 MB is almost always enough for your average Wii owner. =)
Hope this helps, friend.