Wire Diameter to Fabricate a Stevens 820 Slide Lock Spring?

I couldn't find a Stevens 820B forum so I hope this is OK in here. If not please move it.

I picked up a nice old Stevens 820B 12 ga in a trade for about $50, with a gnarly looking vintage Polychoke on it. It needs a new firing pin and slide lock spring. From photos it appears that the spring is made out of spring steel wire but I don't have even a piece of the original one to measure. Any idea as to what the best diameter would be to make a new one with?

Also, any sources for replacement firing pins for less than the $25 and shipping Gun Parts Co wants for a repro? (I am guessing a new repro would be better than a used one, considering the "collectible" value of this shotgun.

Kinda' doubt you'll find them any cheaper!