try taking output from any switching element.... especially thyristors can source a lot of noise... else try ADCing a pseudo random sequence it is a good source of noise...
I've seen zener diodes and reverse polarised BE junctions folowed by amplifiers used as noise sources in some simple test equipment (home made).
For better performance use a pseudo-random binary sequence (PRBS) generator connected to a DAC.
Look in The art of electronics - Horowitz, a chapter is about PRBS generators using feedback shift registers. I dont know if there is something about analog noise generators.