I guess by telerikQuickStart
you mean the namespace used in the XAML out Telerik WPF demos application. If so, keep in mind that we do not provide this namespace as an assembly. It is actually a project in the demos solution which is used as a helper for the application.
You can find its implementation and get its code, if necessary, from the solution which can be found in the UI for WPF download page under your Telerik account. You can navigate to the download page by opening the UI for WPF section in your account and click on Browse all product files. The file with demos is called Telerik_UI_for_WPF_[version]_Demos.zip
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