Unfortunately RadComboBox does not support load on demand out of the box. You can find attached a simple example how to create load on demand using the google suggest web service. You will have to implement the IAutocompleteProvider interface the same way if you want to use other service or direct database access.
Still due to some limitations of RadComboBox (it does not have CurrentText property that contains the typed, but not autocompleted text) you cannot use both autocomplete and load on demand at the same time.
Please, let me know if you need additional help with this.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items