I use a spring steel assortment from Brownells. Instructions for heat treat are in the Brownells catalog I believe and are included on my packaging(it's pretty old though). I am assuming your going to make a V shaped mainspring. This is a fairly straightforward proposition. If the old spring is there for a pattern, simply duplicate it using the same thickness of stock from the kit. It is quite soft and really easy to file and polish. Polish the material well as scratches can become stress concentrators and cause failure later. After the spring is shaped and polished it can be bent. Usually they can be bent cold but sharp bends may require heating to accomplish. After the forming is done the heat treating process is completed and the spring compressed thru a full working cycle to assure it will function. A bit of re-polishing, install the spring and test. The steel I use is oil quenced and cleaned. It's then tempered by slowly bringing to a blue color. I use propane torches for most smaller springs and some used(re-cycled) firebricks to build a temporary oven for the tempuring process. If you don't feel comfortable you can also try looking for the part from the maker or perhaps GunPartsCorp..
