I am looking for a discontinued set of ground lights "EL644BK" model number by Hampton Bay

Discontinued items are rarely found unless Home Depot has a clearance table.

With both HomeDepot and Lowe's you can be sure of onething, when they are out they are out. This is a Home Depot store brand made for them by several different companies. A HD service rep should be able to check the computer for any left in any other store. I rarely see the same set of lights 2 years in a row. You'll have better luck if you just need a part, or do you need the whole set?