His best friends Hermione and Ron would most likely support him. Hermione with her intelligence could come up with great strategies, and Ron, being loyal, would stand by Harry no matter what.
Some of the professors who believe in fairness and freedom might also support Harry. For example, Professor McGonagall, who has always been a just and kind figure. She might see the value in Harry's cause and help him, at least secretly, in his rebellion.
The members of Dumbledore's Army could be on Harry's side. They have already shown their willingness to oppose the bad forces in the past, so they would be eager to join Harry's rebellion against any form of injustice in the wizarding world.
In this 'Harry starts a rebellion' fanfic, it could be related to the treatment of non - pure - blood wizards. Harry, having faced discrimination himself, might start a rebellion to demand equal rights for all wizards regardless of their blood status. He could organize protests and use his influence to make the wizarding world more inclusive.
Hermione might support Harry if she also sees some flaws in Dumbledore's leadership. She is very logical and if Harry presents her with solid evidence, she could side with him. For instance, if Dumbledore's actions are putting students in danger in some way, Hermione would want to protect her fellow students.
Hermione would most likely support Harry. She's always been his loyal friend and would see the injustice of the Daily Prophet's actions. She could help him research laws in the wizarding world (if there are any regarding slander or privacy) and also use her intelligence to find ways to counter the newspaper's claims.
Hermione would surely support Harry. She's always been by his side, and with her intelligence and knowledge of magic and wizarding laws, she'd be a great asset. Ron would also be there, providing his loyalty and his own brand of courage.
His friends from Hogwarts like Hermione and Ron would most likely support him. They have always been by his side through thick and thin in the original story, so in the fanfiction, they would continue to support Harry in his fight against the Ministry.
The Order of the Phoenix members might also back Harry. They've fought against the dark forces before and if they see that the Ministry has become a threat, they'd side with Harry. Dumbledore's old allies in the Order would believe in Harry's judgment and join him in his mission to destroy the corrupt Ministry.
Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.
Some of the younger students who looked up to Harry, like Neville, could support him. Neville has grown to be a strong and brave character. He might see Harry's actions as a way to make the wizarding world a better place. Also, those students who have felt wronged by Dumbledore's decisions in the past, perhaps in terms of school policies or safety, could come forward to support Harry in the court case.
Dumbledore could potentially be a source of help. He is extremely powerful and wise. He could have given Harry some advice or a special magical item before the encounter with the dragon that would prove crucial in Harry's ability to kill the dragon.
His friend Ron could come back to help him. Ron is always loyal and might not want to leave Harry behind. He could bring his family's flying car again to pick up Harry.