1968 318 Cuda Idle return spring recommendation

I need an idle return spring for a 1968 Barracuda with the Carter BBD 2 bbl on a 318 V8. Any recommendations on where to get one?

I had been using an off the shelf universal spring but it's loosened enough to where it won't fully shut the throttle when I gently take my foot off the gas and that's a hazard. For now I just slapped on 2 springs so it shuts but that's not a long term solution.