My most profound apologies. I managed to get that the other way round.
They exist in the pro audio world, but they are not necessarily audiophile quality. Try the Edirol brand (made by Roland). They used to have a really cheap cable product called the Edirol UA1D, with a little D/D converter which did SPDIF I/O for both Toslink and coax, and plugged into your USB port at the other end (I think it was OK in all directions - like a universal digital adapter of sorts but it was meant to plug SPDIF output things into your PC. They probably make a new version now but most of recent audio interface tools tend to have toslink-in, not coax. The UA1D can probably be found used. Otherwise, if you really want to have coax-in USB-out, you might try one of the $200-300 range converters (which tend to have more bells and whistles), or you could go real low-tech and do a SPDIF coax --> MIDI converter, and then add a MIDI-->USB converter cable to the back end... not pretty but...