I would add a servo to something like this http://www.amazon.com/In-Line-Air-Flow-Regulator-Gage/dp/B00173AZ40
as it will likely be a lot cheaper than finding a ready built controllable pressure/airflow regulator. It won't be supper accurate, but if you add a pressure/flow sensor after the valve you can implement a closed loop controller on the Arduino and probably get fairly good accuracy.
A pressure regulator, like the one I linked above will only control pressure, not air-flow. Think of it like a voltage regulator. It will make sure you always have a constant (voltage) pressure on one side, but that will only equate to a constant (current) air-flow if there is a constant resistance involved. True volumetric airflow valves do exist, but they, like current limiting power supplies, are a lot more complicated and more expensive.
Alternatively, if you could live with a few discrete airflow options you could build the pneumatic analog of a resistor ladder adc (google that) with a bunch of different sized tubes (resistors) and a handful of solenoid valves. I probably wouldn't really do it this way, but it sure would be cool if someone did.