Most gambling machines (so I've been told) use a multiply recursive pseudo-random number generator coupled to some mechanical source of noise. For example, the PRNG could be always running, but the results modified by the timimg between button pushes by the player and also modified by some oil-damped-piston coupling. These need be only unpredictable in a reasonable amount of time rather than be exhibit truly random behavior. Getting the statistics correct is trivial. Another choice would be to take a real-time-clock as the plaintext and run it through DES; this makes a slow but pretty good PRNG.
I would guess that in gambling it is more important to eliminate the possibility of cheating than it is to have mathematically perfect randomness.