Telerik UI for WPF[
^] I have found to be very good - Their UI toolbox has over 110 WPF controls and one-on-one & forum support is very good.
DevExpress WPF Controls[
^] is another.
And lastly,
Syncfusion Essential Studio for WPF[
^] is another. They also have a
FREE Community Licence[
^] that gives you access to 800+ Free controls and frameworks for .NET (Windows Forms, WPF, ASP.NET MVC, ASP.NET Web Forms, LightSwitch, Silverlight, Windows Phone, WinRT, Windows 8), iOS, Android, Xamarin and JavaScript platforms. Did I also mention that it was FREE? (conditions apply)