As mentioned above, rubber bands work really well too. You may also find that you don't need them at all. I have one machine without any fitted and I can't really tell the difference. Maybe it will be required when the rod joints wear down, but that's not happened yet.
My test springs were from an old spring-kit sold here in the UK (Maplin) for a few £ the set.
My test springs were from an old spring-kit sold here in the UK (Maplin) for a few £ the set.