RadAutoCompleteBox was introduced in the Q2 2012 SP1 (Version 2012.2.724) release of Telerik RadControls for ASP.NET Ajax. You can download this older version by following the steps given below.
1. Log in to your Telerik account.
2. Go to Products & Subscriptions -> Trial Downloads.
3. Find RadControls for ASP.NET Ajax and select the Download link next to it.
4. On the page with the latest version select All product versions link on the right menu.
5. On the opened page select the version you want to download and download the Automatic Installation package.