If you live near an Apple Store, take it to them and they'll install the wireless/bluetooth option for you for $105 (when I did it) in a few hours. I think that's your best option.
Otherwise, you should be able to get a USB-attached wireless interface. Finding one that supports the Mac might be a difficult proposition. Make sure you do research to make sure that the manufacturer supports the Mac before you buy one. I can't find one immediately.
The other option is an Ethernet-to-WiFi bridge. This would plug into your Mac's ethernet port and turn it into a WiFi port. This shouldn't require Mac-specific drivers, but I'd doublecheck on that first.
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Bitt Faulk