I couldn't quite catch the reason for RadToolBar not being visible in the scope in which you call
$find("<% SearchResultsTools.ClientId %>")
Hook up to your RadToolBar
event and save your sender.get_id()
client-side ID in a hidden input. Now in your button click event, you should be able to reference your hidden input and read its value, giving you the client ID of RadToolBar to use with $find().
the Telerik team