This being raspberry pi absolves you from needing to buy a separate hardware noise generator: it has plenty of GPIO. For example, one can obtain entropy by sampling random noise generated by reverse-biasing a junction in a cheap pn transistor. Here is an example: http://holdenc.altervista.org/avalanche/. Bonus — maybe it will get you hooked on electrical engineering!
Btw, some versions of raspberry pi already have hardware random number number generator accessible at
/dev/hwrng.
Btw, some versions of raspberry pi already have hardware random number number generator accessible at /dev/hwrng.