Relievers come from the darndest places. Many times they are failed starters, such as Wade Davis, who went from being a terrible part of the Royals rotation to one of the most dominant relievers in club history. But you can find them through all avenues - the Rule 5 draft (Joakim Soria), minor league trades (Jeff Montgomery), or as undrafted free agents (Dan Quisenberry).
Scott Barlow was signed as a minor league free agent after being blocked as a mediocre starting pitcher in the Dodgers organization for several years. After getting his feet wet with 15 innings in 2018, he became a part of the bullpen in 2019 and 2020.