Previously there were no problems with getting plugins as PocketMine Forums was working. But in recent years the old PocketMine site constantly had some failures, and finally it ceased to exist at all. PocketMine Forums stopped working even earlier. Players were getting plugins from PocketMine Forums saves on WebArchive.
But there's no problem getting plugins now. There is NostalgiaForum, which has both reuploaded plugins from PocketMine Forums and new plugins from server creators/developers.
You can quickly find any plugins you need:
https://www.nostalgiaforum.xyz/index.php?resources/categories/7/
https://www.nostalgiaforum.xyz/index.php?archive/plugins/