you can search for affiliates in your industry and reach out to them. your backlinks should be from quality websites that are related to what your site is about. stay away from "spammy" sites too, those can actually hurt your ranking. when you email the webmaster, your email should be well composed and grammatically correct, you're building a business relationship so keep it professional. with link it's always quality over quantity, you can use PR score, mRank score and your own judgement to determine if a site is worth pursuing a link from.