I asked the same question a few weeks ago with not too much response. It's what I plan to do though. I'll post a few links at the bottom of ones that I was looking at. There is an interesting issue with certain chipsets within the expresscards which can cause problems, but I'll let you find what works. I can't remember necessarily what I found that was wrong with any of these, it was so long ago, I'll post back when I remember. Anyways, here's a few that should work well. The only big compatibility issue that I found was with Seagate Free Agent Pro external hard drives and the Sonnettech expresscard. It seems there's something odd about the oxford chipset in that one particular Seagate Drive. Otherwise you should be good.
As a post note, I saw a post on a blog this morning that said that the Macbook Pro uses a USB controller for the expresscard slot, which I don't think is true, but I'm not certainly. Can anyone verify this?
Here are the links:
http://www.newegg.com/Product/Produc...20express%2034
http://www.sonnettech.com/product/te...express34.html
http://www.eblazingtech.com/servlet/...ATA-for/Detail