A High Pressure air tank would be the most versatile choice. I personally use the Dye Throttle.
As for what size that would be is down to what is comfortable/suitable for how/where you play and how big you are? Eg. I'd recommend a larger tank for a bigger player and/or a back player. A smaller tank for a smaller player and/or a front player.
As captain bitchslap said mate.
Also I see your new to the nt range, you should keep captain in your thoughts, he helped me out no end when I had my nt.
They can be hard to setup and he's invaluable in the advice department plus he charges an extremely reasonable fee for a full service.