web app - What's good UX for selecting an item from a list of potentially 20k+ items?

Each client using the site has different options in that list, gotten from the client themselves. A normal select field works for most clients (who have between 5-10k options). A few, though, go all the way to 20k+ which causes the browser to lock up for a bit when the page loads.

What would be an alternative to a native select field for showing that many options?