You will need a spring from another master cylinder. The hardware store springs probably will not work right with the pressure valve at the end of the master cylinder. The spring needs to fit just right on the valve to keep a small amount of pressure on the wheel cylinders. Some of the other stove bolters should have extra parts laying around. Before you spend a lot of time looking for parts I would hone and measure your current master cylinder and see if it's worth rebuilding.
