My favourite low-parts-count RNG is a three-component free running 555 oscillator like this:
http://www.eleccircuit.com/alternate-555-oscillator/
you then hook pin 6 of the 555 into an analogue input and read that. Pin 6 taps into the timer's analogue R/C circuit, so you are reading the charge on the timing capacitor, which is always changing, and changing in a pattern that is not linked to the Arduino clock.
If you have at least 12 volts available, you can use a reverse biased B-E junction circuit to generate noise, but in my applications, I only have 5V and building a boost converter takes more parts than the 555 oscillator way.