The format is fairly new; it's basically a faster replacement for PC Card (PCMCIA). A quick search online only turned up one Expresscard-to-Firewire adapter, from Abocom; I'm not sure if it's out yet, since I haven't seen purchase links.
Also, be sure that you're getting the right kind of Expresscard device. There's two formats: Expresscard/34 and Expresscard/54, and if the slot on your laptop is only of the 34 kind, you may not be able to use one formatted for the 54 slot.