I have found a source for Parker springs (Beartooth Woods) but I need to find a source for the roller ball springs. Them little buggers just seem to get away from me.
I just go to Ace Hardware and purchase 1/4x1.5 long springs from the bulk drawer. That's enough length to make 2-3 pens per spring. A quick snip will make them any length you want. They are extra good for custom pens or closed end pens because the length of the hole is less important.
Same as Jeff. The hardware store. Cut to length and now I glue the spring to the finial. Lost one from a Baron at a show so did the quick fix and now have extras in the parts box.