After you get your Instagram set up, start religiously following small and mid-size businesses on there. With every one, check their site to see if they has a brand ambassador or sponsored rider program and when they take applications. (Some may not have them set up yet, but might in the future.) Most small and mid-size businesses will probably only have brand ambassador programs, but as your following grows, bigger businesses will stumble across you. If they have open calls for sponsored riders, being able to list a few brand ambassadorships on your application will make you look good.
Otherwise, if you're hunting for a major brand, you'll have to start showing at major shows and wait for them to come to you, as that's how many of them do it as far as I know.