We can get customer by custom attribute in magento with a single line of code as shown below, Just pass the attribute name and the desired value

<?php
$customer=Mage::getModel('customer/customer')->getCollection()->addFieldToFilter('username', 'yourusernamehere')->load();
?>

In the above code username is the desired custom attribute
and yourusernamehere is the value of the custom attribute

in $customer the customers matching the criteria will get as an array

hope this helps you to get customer by custom attribute in magento