Unfortunately you won't get anyone with magnetic powers until towards the end of the game. Its often a good idea to just concentrate on beating the story of Lego games and save the collecting for after you beat it and have access to all character abilities.
I've yet to even get Polaris, and I've finished the whole story and have done all the "side story" missions for up to having collected 150 bricks. --- You're only a cheater in the eyes of someone who has yet to gain the knowledge to do what you can do.
Someone said that she is near Central Park, but I haven't tried looking for her yet, nor do I know at what point in the story her icon appears on the map. --- "Man how to people figure this stuff out? A rubber band on the R button." - HYPERMECHA
You don't need to rush it. Just collect things that you have abilities for and then once you do end up getting Magneto you'll be able to get the things you couldn't before. --- You're only a cheater in the eyes of someone who has yet to gain the knowledge to do what you can do.