When I looked at the slugs at the MC Swimbaits booth at the Fred Hall Show they came pre-made with the springs already in them. As far as I know the springs don't serve a purpose other than to keep the lure from sliding around on the hook. This problem can be solved quite easily with an application of cyanoacrylate (AKA: Superglue
). Yeah, it's a little more permanent than having the springs which in theory should allow you to remove the hook and switch slugs easily, but the remedy for that is have a few different colors pre-rigged and also carry a tube of superglue in your tackle box. The springs aren't worth bothering with IMHO. If you wanted to try them though you could look at a teacher supply store or something.
Good luck with whatever you choose to try. Go get em!
Cody