Here is a wonderful site with free downloadable artwork of koi.  The Clip Art Queen will help you with your koi drawings needs.  
 
Koi fish drawings and sketches of koi fish are intriguing on so many levels. The koi fish are brilliantly colored with shades of gold, orange, red, black and white.
They have a surreal or mystical quality that naturally unleashes creativity. This is why they are often seen depicted in shades of blue and green, even though they aren't naturally available in those shades.
 
 
Colorful Koi Fish Drawings (clipartqueen.com)