I believe there's a huge literature on methods to generate pseudo-random numbers. Experts on HN please jump in. One approach, which I thought is quite ingenious, is to observe cosmic radiation and extract randomness from this data.
* Unless you can figure out where cosmic rays come from and model their propagation mechanism accurately, in which case you'll solve some long-standing astrophysics questions.
Also, if you need random numbers there are usually better ways to get them.