From 'Kubla Khan', of course. Ted Hughes, in his brilliant and batty introduction to his Selection of Coleridge's verse, dilates upon his theory that the 'alph' here is the letter, and short for 'the alphabet' as a whole, and that the reference to 'alph, the sacred river' is STC's way of invoking poetry itself as flowing from this exotic source. (This is not a theory original to Hughes, of course; although it was there that I personally first encountered it).
Now, this is a bit of Coleridgeana that is arcane even by my standards (and, as this blog has shown over the last few weeks, I'm no stranger to abstruse Coleridge references), but in the light of this notion, I mention only this: that Johann Albert Salmon describes John Chrysostom’s writings as flumen sanctus eloquentiae, ‘a sacred river of eloquence’ [Johannes Albertus Salmon, Apologeticum Tentamen (1788), 44]. Might Coleridge have seen the phrase there? 'Kubla' is a poem uttered from a golden mouth, if ever poem was.