Any steel, brass, copper spring will
Work.
Home Depot, auto parts store, etc.
The problem probably lies in the firing mechanism.
Disassemble the switch and re assemble taking care to observe the action of the button to be sure it's doing it's Jon and making contact with the bottom of the battery.
Install the switch and separate the top and bottom tubes.
Drop the battery in.
Fire the button and feel to be sure it's making contact.
If all is well, attach top part of mod and atty, making sure everything has a solid tight contact with the battery.
Fire the mod.
If it still doesn't fire, it's most likely cursed by a demon from hell.
Toss it or call a priest.