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i don't know where you can buy one, but there are different types. there's the dongles which can be cheap, but some of them overheat.

if you get PCI one, it shouldn't be hard to install, but I don't know if it has to be OS X compatible.
 
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So thanks for advising me on getting the D-Link. I got it in today and it works awesome! Now I can use my wireless keyboard and mighty mouse. :):)
 
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