Yes Daniel, Pinball Resource has them.

SP-100-314 Bally drop target spring for standard DT Bank $0. 60 cents each
SP-100-350 Bally drop target spring for inline DT bank ( Paragon etc) $0.60 cents each

As you see they are hugely expensive NOT!!

If no ones doing a PBR order soon I can bung a few in the post for you.

Incidently I think I did read a thread on the operations of these DT banks and I'm sure someone said that when the round rod at the bottom of the bank gets a bit of wear in it ie: when a groove starts to get worn into the rod after 1000's of operations this can affect how easy or hard it gets for the target to drop down. Might be worth thinking about getting a new rod if the old ones a bit worn.

Thanks Chris
  I never think of pbr for Bally bits have not even looked at the drops yet stripped the PLAYFIELD then got busy again lol