Usually you get emails from advertisers, but if you're not getting emails from the brands you'd like to work with you can reach out to them. I have a few guides on reaching out to potential advertisers. Another thing you should do is join blogging networks (i.e. Influencer networks) because they either make you pitch to brands or have brands pitch to you. Another road to go to if you're a larger blogger: join an agency and they will represent you.