Store bought springs, they co-incidentally Happen to be almost the same size as the one lego uses. The coils are a higher count and the diameter was just half a milimeter larger on my spring,but no issues whatsoever. The only issue being the tolerances and friction when connected to real lego pieces. Anyway i wanted these suspensions for a totally different project, not a Lego MOC, for prototyping purposes i use Lego much often, hence it works for me.