Sidebar Issue

OK, I have a few (that will soon be several) themes available for my visitors to choose from. Because I change my sidebar often, Id like to call it from a centralized location instead of from inside the theme folder so that I only have to upload it once each time I make a change. What I mean is, Id like to put one sidebar.php in say the themes folder and call it from there to all the themes. I tried using a standard php include:

<?php include("domain.name/wp-content/themes/sidebar.php"); ?>

but just get errors. I hope Im making sense. Thanks for any help!