You're in Japan? Hit a Hard Off or similar recycle shop and dig through the bin of PC cards (most locations have one back in the junk section). You may luck into an Orinoco card, but otherwise you're looking for a Buffalo AirStation WLI-PCM-L11 and it shouldn't cost you more than 300 yen for one. This card is 802.11b and should support up to WEP encryption. I believe the AirStation Install Disk is still available directly from Buffalo. Make sure you get the correct model because any of the later cards with WLI-CB in the model number will not work in your computer at all (they're CardBus and your computer is not compatible).
While you're digging through the PC Card bin, the TDK LAK-CD021AX and CD031 Ethernet cards should also work for you, as should some of the specific 3Com cards mentioned above. Just be sure to find the dongle too.