There is a costume in the garage. To open up the garage, there is a red button you must wall ride on the left side of the second warehouse. Once you open up the garage, go to the far end of it. Transfer from the quarterpipe to the raised quarterpipe, then jump off of the kicker and grind the transformer (the large black object). Once you do that, it will transform into a halfpipe. On one side of it should be a Floating Helmet icon, Trick off that side and get it.