Find some one like a gunsmith who will explain how to temper thin spring steel in hot oil once you've cut & shaped it to the right size.
See this eBay number for the sort of thing available for the annealed spring carbon steel strip .. there's loads of different sizes & no end of places to get annealed spring stell ready for shaping & tempering
This is a small selection of my spring stock. All of the flat (sheet) is in the annealed state so very easy to make your spring to whatever shape and form the just heat treat it. Hey presto! A spring: