Some weeks ago I made a visual tool to create a bitcoin private key. The most secure way for key generation is to flip a physical coin 256 times. I flipped a coin 256 times, and made a video how to create the bitcoin private key with my tool.
It's certainly not perfectly random but it's safer than relying on so called "True Random" Number Generators that can be bugged or having design weaknesses, and are actually just trustful blackboxes.
If you wish a guaranteed and independent entropy then you can buy any uranium mineral and use Geuger counter to make as many random bytes as you want.