You can buy very nice American made chrome plated acorn nuts in varying sizes at Orchard Supply for one.
They may be available at places like Ace Hardware and True Value as well.
I don't think Home Despair carries them, most times their bolt selection is a touch pitiful.

In my neck of the woods they sold out fast cuz the Harley gang found out about them.
Since they were about a third the price of what looked to be the same thing at the Harley shop....

If you find a store that carries them, find out what day the bolt deliveries come in.
Give em a few hours to get em in stock and you'll probably be able to find what you want.

Not exactly what you asked, but I'd rather have a real nut than a cover.