Looks like Home Depot still sells them in store if you're only looking for the basic slide on style used with newer striplights.
Plunger endFixed endYou'll need both the plunger end and the fixed end to form one complete set for a lamp.
If you're looking for other styles such as bolt-on, butt mount, turret, etc then eBay or
Semperlite is probably a better option. Note that if you want to fit them to a strip designed for bi-pin lampholders, you'll have to widen the opening.