Hindley Electronics in Bowling Green Ohio is probably the best in our part of the country. Their number is 419-352-7110
or try equipment parts wholesale.
You can buy replacement thumb sticks here:
http://secure.llamma.com/catalog/index.php?cPath=73_97
You can choose between gray or black thumb sticks and even PS2/3 style sticks.
You can buy the replacement analog joysticks from Spark Fun.com here:
http://www.sparkfun.com/commerce/product_info.php?products_id=9032