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I've completed Superchargers's story mode, but I'm still missing a lot of mods. Does anyone here know where I can get mods quickly and easily?

Thats a good question. What I did to get my vehicle mods was playing the very first land vehicle zone, which is in The Rift to Skylands chapter 2. The zone is short, mod boxes are in plain sight and there's 3 mods(? Maybe 4) Once the vehicle zone was over I just restarted the level. I grinded this level for wave 1, after the later waves released I had enough mods by then not to care for grinding anymore.
I'm sure there is a better and more efficient way of obtaining mods but this is how I got most of mine. I hope this helps with your lack of mods!