I remember a guy back then when I still regularly check the Digital Goods section. He had around 12k followers on twitter but no one wants to buy his account because when you do a twitteraudit of his account, there's just about 2k+ real followers. He probably used many websites similar to what you and others here are suggesting. If you already have around 100 followers, I suggest that you just keep on following other people and hope that they follow back. Using any website that gives you automatic followers will just ruin your account.
I have 6K followers on Twitter and I am also following 6K people because I do follow for follow. I follow back all the people that follow me and unfollow those who don't.