I use 3d printed springs that I made some time ago. Here you have the link:
You just need some pen springs and some nails. The rear shocks have one spring while the front shocks should have 2 springs. I recommend 2 springs in the front shocks because there is more weight in this side. You could also put harder springs if you have. To put 2 springs in the same shock I introduced the first one inside nail and inside the part called "cuerpo" and the other also inside the nail but between the parts called "cuerpo" and "anclaje". I don't know if I'm clear enough. I'll try to post some photos to make it clearer.