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Pretty much ANY PCIe USB card with a NEC chipset will work without additional drivers.
No need to look for one that's explicitly "Mac compatible".

Edit: Sorry, just noticed you picked a NEC-card. So, just find a PCIe one.
 
Cool, so using this pic as an example, does it make a difference were the card goes;

http://macbitz.files.wordpress.com/2008/10/img_2023.jpg

i.e. 4 or x16

and can anyone link to a place online to purchase?

Thanks

Typically you would use the x16 speed slots for data hungry devices like video cards / raid cards, but it wont make a difference to the usb card which slot it lives in (x4 is enough bandwidth to max it out).
 
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