Ask the user to hit the spacebar a few times. Use the time in microseconds between keystrokes to seed your random number generator.
(or better, use the remainder of the time in microseconds mod 256 microseconds to eliminate any human periodicity. So each keypress gives you 1 random byte, use the appropriate number of strokes to seed your generator)
You could use the System.Security.Cryptography.RNGCryptoServiceProvider.
Most programming random functions are based on mathematical algorithms. There are random numbers generators, that use wave noises in air (from space radiation), but they're hardware based.
Put a cat in a box with a bottle of poison, which will be released when a single atom of a radioactive material decays. Come back in one hour and open the box. If the cat is dead, use 0 as a random bit in your program. If it is alive, use 1.
Repeat with as many cats and boxes as is necessary.
For that, you would need a hardware random number generater.
A good-enough-for-most-cases substitute for this is measuring the timing of "hardware events" such as user input or network activity.
On Linux, the /dev/random device produces bytes based on this method.
I would recommend you:
True Random Number Generators (TRNGs)
http://www.random.org/randomness/
That will point you in the right direction!
It's very difficult to get really random numbers. PRNGs are, after all, in some sense predictable. But it's much more difficult to correctly build a random number generator based on a physical source; most random sources don't produce real random bits all that fast, and it's hard to tell whether you've made a bad blunder. A good PRNG is better for virtually all intents and purposes.
(Only use a high-quality PRNG if you need one, as they're much slower, and make sure you use the right kind if you do: a PRNG designed for crypto is not the same as one designed for Monte Carlo simulation – crypto PRNGs make sure every bit is hard to guess, whereas MCSim PRNGs make sure that spectral properties are good – and most other uses for PRNGs just want them to not be cruddy and to be fast; the standard library one is usually good enough there.)