The reason why some mangas are not available on manga provider sites because:
+they are licensed
The sites where you can download RARE manga for free:
+ stoptazmo - based on my experience, you can download two chapters at the same time for fast download. No limit.
+ animea - you can download here with no limits.
+ sugar888 - if you are looking for the RAREST, then visit this one. You can find some licensed and banned manga here. The prob is you need to sign up for membership and there's a limit of 10 chapters a day.
Well, that's all I know. :grumble: