Assuming your items are sorted alphabetically, simply setting IsTextSearchEnabled="True" should jump to the items starting with the letter (or letters) you type into the ComboBox.
Here is an example of one of my ComboBoxes, I have simplified the bindings as it's clearly not the important part here...
<ComboBox
ItemsSource="{Binding MyObjectList}"
DisplayMemberPath="Description"
SelectedValuePath="Code"
IsTextSearchEnabled="True"/>
This works perfectly for selecting a value from the list, however, the search value you type will not display in the TextBox part of the control as I have IsEditable set to false.
If anyone would like to explain why this has been voted down it would be appreciated, I don't see any problem with the answer I've provided and don't see why I deserve to lose reputation when I'm only trying to help (and have provided a reasonable answer!)