Warren Martineck is one of the arts who has provided sketch cards for the Justice League Archives Trading Cards. He’s recently posted this great double panel of the JLA and JSA teaming-up to fight Starro. These are Artist Proof cards, which he describes as:
An Artist’s Proof card is a separate blank card with special printing or with “Artist’s Proof” printed on the back, given to the artist so they can sell, keep, or do whatever with it. It being blank, the artist can draw whatever he wants on it or be commissioned to do a specific subject. Though they usually have to be approved by the company before being sold.
He still has some left so contact him if you want to commission your own Artist’s Proof.