even if you find a soft keyboard cover they are still a complete pain to type on and are generally not worthwhile.
a generic cover like the aforementioned one is not going to protect against spills since it doesnt cover the whole board.
if you must drink at the keyboard perhaps use something like this
http://www.techpin.com/wp-content/wireless-silicone-keyboard-2.JPG for when you have a drink in front of you (although they suck to type on) or one like this http://www.sealshield.com/ but they arent going to be the same as your keyboard.
i like to have something to drink on the table as well and i've also spilled many times. i ruined a 4 month old $130 wasd mechanical this way.
what i find best is to keep a drink (if its a glass) as far away as you can so you have to reach forward to grab it. if its a bottle i would suggest turning away from the desk to open it if you arent sure if it was shaken up or not.