I have complied a list of the best free Kit Bashing Libraries
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Poly Workshop
https://cubebrush.co/polyworkshop/products/audboa/free-sci-fi-kitbash-set-fusion-360-zbrush
Nick Govacko - No longer avaiable
https://www.artstation.com/artwork/kit-bash
The French Monkey
https://gumroad.com/l/OUUsD
Andrew Averkin
https://gumroad.com/averkin
Adrian Rutkowski
https://gumroad.com/adrianrutkowski