The Elder Scrolls: The XVII Princes of Oblivion

[Arutema]

I believe you can only get Bound Bow from a specific bandit camp or something.

Looking into it, no apparently there is a guaranteed drop in Fort Amol (which is where I typically get it), but a few merchants do sell it: Calcelmo (Markarth), Phinis Gestor (Mage's College), and Falion (Morthal).

IIRC, as it's an Adept-level spell, they'll only sell it once you hit 40 in conjuration though.