You'll probably save time (and maybe money as well) simply by buying a new stapler! But much more satisfying if you can mend your old one.
A bit of research indicates that there are instructions online for making spings, such as http://www.instructables.com/id/Make-your-own-springs-in-seconds/
If you search eBay for spring wire you should find the materials you need, in fact you might even find the right spring, or a longer one you can cut down.