Series can be a singular or a plural noun, depending on its meaning. When it is used to refer to a single set of things, it takes a singular verb even if it is followed by the preposition of and a plural noun: A series of medical tests is planned for next week. When series refers to two or more sets of things, it takes a plural verb: Three series of medical tests are planned for next week. Example in these phrase: "several series of books" "a series of books" . but in your sentence "They're series?" the word "they" refers to "outsourced materials" and is correct in plural form. The end