It is possible to make this machine wireless but you will need to track down an original Apple Airport card. The original cards use the 802.11b spec and transmit at around 11mbps, kinda slow compared with the Airport Express.
Unfortunately, the slot that you put the Airport card into, although it looks just like a PCMCIA slot, is not. As you have already noticed, there are no external slots on that machine.
The ONLY card that works in that machine, is the original Airport Card.
Google for one, or eBay, but expect to pay in the region of $100 for one.
Due to the increasing school demands from my daughter, I'm wondering about the possibility of putting an Airport card in our iBook G3, graphite; 466mhz, 320ram, 10gig hd. Apple has a refurb' at $39 bucks listed; right price, but it's an Airport Extreme, and I understand that the G3 would not handle that model. So thinking perhaps about a PCMCIA wifi card, but should it be the 'B' card or the 'G'? There seem to be several brands, Buffalo etc; and which is best recommended? Still working on a Panther upgrade here; emailing to & fro with AppleRescue as to why their disks wouldn't boot in this machine, but at 10.2.8 currently, perhaps nothing lost. Thanks, Eric