There are various sources. I don’t know them all (maybe Michael will tell us what they all were, if he even keeps track), but one of the largest is the Tatoeba Project, of which a large percentage of the sentences come from the Tanaka Corpus.
Original was Tanaka Corpus. Maybe 30,000 of them have been made specifically for renshuu.
On top of that, 10,000+ of the original Tanaka sentences have been revised, and I also have a native speaker/teacher going through those remaining original ones and marking them as fully "ok" or needing improvements.
Ah ok! thank you both for the info. If im not mistaken the ones from the Corpus should be writings from native Japanese sources which is excellent, and i would assume the 30,000 made-for-renshuu ones are also of japanese native source too, so thats actually really cool to know and raises the credibility and accuracy quite a bit. Once again yet another A+ aspect of renshuu! Thanks again guys and keep up the good work!