Door Track Spring - Need Replacement

[CRW-FK5]

I was putting the window back together in my '70 and noticed a spring rusted and broken inside the rear door guide.  Are these being reproduced anywhere, perhaps in a kit?

[Lennard]

I would just go to a hardware store with the old spring and find one like it.

[Bronzedodge]

I'll second that.  True Value or maybe an Ace Hardware.  Other option is to find a spring supplier online, maybe McMaster-Carr.   

[CRW-FK5]

Thanks, guys.  I'll do that.

[Mytur Binsdirti]

What exactly does that spring do. I've seen them, but can't figure it out.