There are a number of ADC options out there, at a number of price points. I'd look into options mentioned not just here, but also at Computer Audiophile, and Audio Asylum.
One example that has gotten some press lately: Furutech (GT-40, I think) has an ADC-DAC that has gotten good reviews for around $500 or so.
One example that has gotten some press lately: Furutech (GT-40, I think) has an ADC-DAC that has gotten good reviews for around $500 or so.