Discovering which PCMCIA Wifi card can work on OS X Leopard
There is a list of such cards on Low End Mac up to 10.4

But I believe better way would be just to go to ebay for native Airport Extreme if your particular model is aluminum one.

If it is Titanium then CardBus is the way to go since original Airport 802.11b cards are rare, pricier and don't support WPA2.

P.S. And I wouldn't recommend 10.5 on Titanium Powerbook