Hello, hopefully someone can help!

I recently bought the Logitech Z5500 Digital speaker set up, which is great, and i can connect it to my Ibook G4 via the 3.5mm jack, but to get the proper use out of the speakers, i need to convert USB to coax or optical (Toslink)

Which do you think is a better option, and where can i buy a converter for cheap (the Maudio Transit is a bit much!)

Thanks guys!