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You can replace the internal WiFi card, just check eBay and use your model ID to figure out which one you need. Since pretty much every Mac has an internal WiFi card I can't see any manufactuer being bothered with building an OSX driver for a USB WiFi card with no actual market.

iFixIt.com can show you how to replace the WiFi card, often including the part number.

This link may offer a solution.
http://tonymacx86.com/viewtopic.php?f=169&t=28845&start=10
 
I should say I would like a USB one as I also have a windows box that does not have a wireless card so if I can merely swap between the two machines when needed, great.
 
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