This poem, sadly, did not appear to be available on line - and that, as Rambling Ducky has so kindly pointed out, below, is because Campbell never wrote it. Rambling Ducky tells us that no one ever wrote it, and that the poem is nothing but a figment of Wikipedia's imagination.
Ah well! We still have Campbell's splendidly tragic ballad Lord Ullin's daughter.
Here it is.