You can get the Telerik.Web.UI.Skins.dll corresponding to your framework version from the RadControls installation folder which is usually C:\Program Files\Telerik\RadControls for ASP.NET AJAX Qx xxxx\
If you want to download the DLL here
are the steps. Once you choose the appropriate version ,you can select the DLLs, Scripts, Skins and Resources only
package from the opened page.