Avalanche Breakdown Noise as digital input
Start by AC-coupling the signal, then amplifying that.  You could then use a typical data slicer, which compares the instantaneous signal to a low-pass filtered version of itself.  Then you sample this digital signal whenever you need a new random bit.

Be very careful that the noise from the digital circuitry doesn't feed back into the analog signal somehow.  If it does, the analog section is no longer statistically independent, and the result therefore not truly random.