Since a fair number of you commented on the Dark Chocolate Espresso Malt Balls -- fabulous in the cookies, by the way -- here is my source. They do mail order, too.
Somis Nut House
Click on Candy, then, in the search box, type malt, and you should pull up 3 kinds of malt balls. ($2.99 for 1/2 pound)
Their nut flours are great, too (hazelnut and almond).