Maybe something like this would work:
Audio Oscilloscope, Spectrum Analyzer, Signal Generator - eBay (item 110248843429 end time May-11-08 06:03:20 PDT)
I'm not sure what it uses for the ADC... a sound card? The screenshot from the Spectrum Analyzer shows -130 db... Maybe if you have a 24bit soundcard that's how you would get that kind of resolution.