Looking it up on the internet and see what company's are looking for a sponsor helps you just have to sell your self sometimes it's not all about how big you jump :)
As Ashleigh says, showing at big circuits helps you to get more attention of sponsor brands, but also is important to have good results.
But sometimes you don't need to be showing to get sponsors! If you have a lot of followers on the social media and you consider yourself an "influencer", sponsors can contact you to talk about their products.
Hope it helps! :)
It helps if your jumping big tracks and placing consistently and are super friendly, helpful & caring to everyone on the circuit. If a business wants to sponsor you they'll get in touch with you.