If you have the Webster plans, the spring information is given on the sheet which shows the assembly of the valve body. The critical things are the inside diameter of 0.188"--Can't be less than that or the spring won't fit over the tapered valve guides. The wire diameter is also given. Free length of the spring is rather subjective. The intake spring has to be strong enough to pull the valve shut, but weak enough that the engine vacuum can pull the valve open against spring pressure. The exhaust valve is opened mechanically by the rocker arm, so it's spring can be heavier. Only other thing is to check and make sure that the exhaust spring, when compressed fully, isn't longer than the lift of the camshaft. Go to your local nuts and bolts store, take a piece of 3/16" rod with you to test for inside diameter of the spring by sliding the spring over the rod. Take a Vernier caliper with you to check the wire diameter. My sources in Canada are Brafasco and Fastenall.
