From the scholia to Aristophanes, Clouds 1198 (where the primary headword, a nominative plural, occurs: web address 1) and 1200 (where the subsidiary lemma occurs). For metaboloi, see mu 682. But note, in addition to the purely ethical aspects of the word focused on here, the possibility that protenthai were actual Athenian officials with quasi-religious functions: see Athenaeus, Deipnosophists 4.171C-D (4.71 Kaibel); Dover ad loc.; and generally LSJ s.v.
For this term in a more straightfoward sense, see pi 2870.