i have noticed some production material(illustration /genga etc...) don't get uploaded here the reason being that they are not legally available online, so is there any other sites that has these things cause i love genga alot

In a lot of cases it's just because people cant be bothered with saving and uploading it. If there is an issue with legality, it will be removed.
(Not suggesting that you take materials shared directly with you by creators for your eyes only, and then upload them without permission.)