Is 555 timer good to generate 20MHz clock?
What you want is a crystal oscillator module like this one from Farnel or the one PeterJ recommended. They come in 4 pin packages like in the images below. They will generate your clock for you.





Both modules mentioned have tolerances of 50 ppm and operate at 5V.

It's not to be confused with regular 2 pin crystal like this below. Those are part of the circuitry of external clocks for MCUs, for example.