Purchase a Behringer UCA202 to add optical out via USB to the Mac. Then you can get a unit with a toslink optical connection. You'll have way more options going that route than restricting to ONLY units that have USB.
http://www.amazon.com/Behringer-UCA202-Audio-Interface/dp/B000KW2YEI
http://www.amazon.com/Behringer-UCA202-Audio-Interface/dp/B000KW2YEI