Originally Posted by GlasPak
I couldn't find AttributeValueSelect when I looked for it.
It is in the [XC5]/classes/XLite/Model/AttributeValue/AttributeValueSelect.php file.
But you should not edit the file directly. Instead create a custom module that decorates the methods of the class that you want to change.
Please refer to the documentation for developers at http://kb.x-cart.com/