Declined with the following description:
Telerik controls can be found on the client-side through the findControl() method exposed by the Telerik client static library (http://www.telerik.com/help/aspnet-ajax/telerik-static-client-library.html). For example:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script>
			function pageLoad() {
				var btn1 = $get("<%=RadButton1.ClientID %>");
				var btn2 = $telerik.findControl(btn1, "RadButton2");
				btn2.set_text("New text");
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
		<telerik:RadButton runat="server" ID="RadButton1" Width="190"
			Height="190">
			<ContentTemplate>
				<span>RadButton Content</span>
				<telerik:RadButton ID="RadButton2" runat="server" Text="OldText" />
			</ContentTemplate>
		</telerik:RadButton>