Here is a solution in the case of trying to find a drop down toolbar button on the client side. You can use findItemByAttribute in this way (based on example in this forum post).
Define your drop down button as such:
and then in your script you can find it (assuming you already have found your RadToolBar as "rtbar") with:
ddButton = rtbar.findItemByAttribute(
This information may be in the documentation somewhere, but it was not easy to find. I did not see any specific example that included how to set a drop down attribute and then make a findItemByAttribute call.