I just bought saw the Rustoleum Spray Grip at walmart for $2.50 and it's amazing. It attaches to any regular spray paint can and it really does work. Some reviews say it isn't the best, but mine works really well so far (update 2 years later and still going strong). It doesn't seem to be the highest quality plastic, but for what I'm doing it's just fine.
I'm refinishing an old mirror from Kirkland's and the finish was very pretty but too traditional for me and I knew I wouldn't use it unless I changed it up.
One full can of glossy white Krylon Fusion and my hand and fingers aren't all cramped up and my fingers don't have runny spraypaint all over.