You're looking for a "flat spring" but you have to also pay attention to the thickness of the spring as well as its material. Copper and copper-alloy springs hold tension differently from steel, and there are also different grades of steel. If your flat spring is the "wrong" ("wrong" here is a function of taste and feel) is used, the tension characteristics of how the key feels to operate, depending on how open it is or how closed, you will get an "arc" of resistance that feels either more or less comfortable (uncomfortable = hang-ups in fingering mechanics). The main 1st concern is the flat spring must be able to hold enough tension that the key cup won't blow open when the horn is being played (you will not be able to play low notes well, or possibly at all, if it does blow open -- often an issue on side keys on old Buescher altos, for example, and pre-1930s King palm keys). After that (a minimum level of thickness is necessary to allow the key to stay closed when playing), the thicker you go the more the arc of resistance will increase as you open the key, given the same amount of downward pressure (pressure to keep the tonehole covered by the cup).
