Foamblast drum springs
The Foamblast drum springs are originally used as replacement springs for Nerf drum magazines. As such they are compatible with the 18,25 and 35 round drum magazines. The 18rd magazine will require some hot glue to keep the spring in place. You can find a tutorial o how to replace your drum spring right here.
These springs are also used for a variety of 3D printed magazines, such as the Gecko magazine or the magazine for the C226 Skyline by Flatten_the_Skyline. They are widely considered the go to magazine spring for similar applications.
Price is listed per spring.