Ron,
Any hardware store should have a spring which would work. It merely supplies a little force to ensure the upper rubber gasket/ring seats on the flange. It doesn't have to compete with oil pressure or anything. Use a metal flat washer between the spring and the filter cartridge to prevent the spring wearing into the metal of the cartridge.

The correct tension should be when you push lightly down on the cartridge it pushes the cartridge back up above the level of the metal can.