Umarex Steel Storm Repair

Is it no longer under warranty? Have you disassembled the gun yet? You may find a comparable spring at your hardware store such as ACE or auto parts store such as NAPA. Just buy a longer one and cut it to match the original spring. Bring it (just the spring - of course!) with you ask a clerk for help finding that size if you must since that's what they are paid to do. Good Luck.
 
Or make your own spring.

If you have a micrometer, you can measure the diameter of the wire. Then buy piano wire of that diameter; then use a drill and a metal rod that matches the internal diameter of the spring or a little less to "make the spring". One end of the wire is held by the drill while the drill is also holding metal rod. You'll have to work out how to do that. Hold the other end of the wire with pliers. Turn the drill on at a slow RPM. While keeping the wire held taut, let the drill pull the wire around the metal rod till you get the correct number of coils going around the rod. Clip the tag ends of the wire and you now have a spring. It takes a bit of practice but piano wire is cheap.