Some people have been trying to sell other people's works (that are free) or used open source codes to create their own checker / generator which is dumb in my opinion.
#1 NodeJS Nitro Generator (Compiled)
#2 Nitro Generator + Checker (based on Python)
#3 Nitro Generator (based on Python)
If you want to dodge paying more than 5-10$ for a nitro codes or generator, it would take you 10 minutes to setup Python and run the scripts that are not compiled, using proxies will boost your chances of getting a valid code faster.
Happy generating, DON'T leech!