McMaster carries some great gas springs:
http://www.mcmaster.com/#gas-springs/=l9zzmr
To know what to get, you need to figure out a couple of things:
- Where are your mounting points?
- What motion does the spring have to go through? In other words, how long does it have to extend/contract?
- How much force is required at that location? Are you trying to use the spring to cause motion in the arm when a catch is released, or are you using it to counteract the weight of the arm to make a motor run easier?
The first two should be straightforward to figure out based on your specific design. The third is more difficult... Ask the physics teacher at your school to walk you through the appropriate torque equations, as in-person discussion is often more enlightening. You know the approximate weight of the arm, but you also need to know the center of gravity (aka the balance point). If all the weight is out on the end, it makes it much harder to lift the arm than if it was all next to the shaft.
Another method to figure out the force you need is to get a force gauge (some physics classrooms have some!). Hook it up at the point you want to mount the gas spring, and measure the amount of force you need to hold the arm up at that point. Getting a spring with that much force will basically cancel out the weight of the arm.