Do not use a drill shaft because they are usually high speed steel a little harder to get the temper right, if you can find a carbon steel drill that would be good. Using Silver steel rod sold in many sizes is probably the best for this part, it is sold in its soft state so easy to file and temper, and if my memory is working on your side of the pond it is called drill rod. Carbon steel for making small parts can be obtained from old files pieces of re bar and for flat springs those disposable panel saws for wood work well so look around there are lots of quality steel found in old tools. The one thing I will say about re purposing carbon steel is that you will have to heat soften it before you can work on it then re harden it to the correct temper but that is not rocket science plenty of how to do it on the internet.
