Suppose I have a list of a couple of thousand organizations and a user needs to be able to select one of them. The list is too large to populate in a dropdown at page load, and the user often knows what they want but it's not the first part of the organization name. That is, they know "Collections" but not that the precise name of the organization is "Department of Collections". So the user will need/want to type in some information.

It's easy enough to use an autocompleting textbox of some kind, but I don't want to allow the user to type in random text - they have to choose one of the organizations explicitly.

What's the best solution?