Try searching for "proportional pneumatic valve" on google. There are many different flavors. A common unit I have used for prototyping air actuated brakes on very large robots is a proportional flow control valve such as:
http://www.ascovalve.com/Common/PDFF...PosiflowR2.pdf
We have prototyped this in our system with general KOP pneumatics and it works well to control velocity of the cylinder.
One lesson learned: Make sure you double check the signal voltage before ordering parts.