What do you think, that there's some sort of used Pokemon dealership or something? Your best bet is to find a friend, seeing as there are 151 Pokemon in Red Version and only a small handful of in-game tra--. . . wait a minute, there's actually a useful answer to this question?! Color me surprised.
a1019 If you evolve Poliwag into Poliwhirl, you can trade it for a Jynx in Cerulean City.