Apostolius 4.33, etc.
Clearly a particular variety of piper is (disparagingly) envisaged in this proverb; certainly not (e.g.) the kind who, from 586 BCE, won the prestigious solo piping competition at the Pythian Games (see West 337). For lowlier practitioners see e.g. the paid piper who was a sort of musical hanger-on of the Athenian eponymous archon each year (Aristotle(?), Athenaion Politeia 62.2 [web address 1]); and if the gloss on the proverb is correct -- a crucial proviso -- others without such guaranteed income had to make their living opportunistically. (For the phrase "for free", e)c a)telei/as, cf. e.g. [Demosthenes] 59.39 [web address 2]). The scenario appears to be one in which the organisers of a sacrifice -- the common currency of ancient religion -- have not made advance provision for the participation of the piper but can be persuaded to hire him (or her) ad hoc.