Glen,
Jimmy's approach works like a charm. And if you want to hang on to the existing spring, you can try out your local hardware store. These are being fast replaced by "home improvement" stores such as Home Depot, but there are still a few struggling to make a living in my area.
Yesterday, I went to Ashton Hardware. In the back was a set of boxes with drawers of springs of all sizes. I found a small conical spring for a battery holder I'm making for my charger. It cost 50 cents.
Good luck with your project.
Paul