Hello, I am searching some sites, even looked at the hardware source section on arduino.cc, but I didn't find out yet where I can buy BUTTONS.. 
I took a very close look at http://www.nkcelectronics.com/ , http://www.hvwtech.com/ and a bit on http://www.sparkfun.com .
Can someone please help me?
Sparkfun has them under components/buttons and switches: Buttons and Switches - SparkFun Electronics
Since you have already considered Sparkfun as a source, you may have overlooked . . .
or
(currently out of stock)
What I like about the long stem is that the button can be activated by pushing sideways on the stem as well. Sometimes that's nice for big fingers.