I am not sure I understand your question fully.
RadRichTextBox targets at supporting rich text editing capabilities. When you refer to doing searches
, do you mean searching within that rich text box or you want to use the control within a from that will do a search?
Generally, there is a control in the RadControls for WPF suite that may be more suitable for your scenario and it is RadAutoCompleteBox. I suggest you take a look at its documentation
and online demo
I hope this helps!