Actually, there are true random numbers on computers. There is system software to gather entropy from non-deterministic events within the computer such as mouse movements, key strokes, delays between network packet reception, etc. This is implemented in hardware in some motherboard chipsets. Unfortunately, these methods don't produce data quickly.
However, you can plug into a computer something like an Araneus Alea I hardware random number generator, which produces random data from the (unpredictable) avalanche breakdown in a transistor. You could plug in a geiger counter and observe the temporal differences between emissions from radioactive material, like they do at HotBits.
TkResolver++ has drivers for both the Araneus Alea I and HotBits.