Just about any place that sells Indian (India Indian) groceries should carry this. It will often be labeled as "gram flour". I remember reading somewhere (maybe in one of Mario Batali's cookbooks) that the chickpea flour produced and/or sold in Italy was different from that produced and/or sold in India.
If the distinction is indeed important, then I'd lean towards the Hayden Mill product ziggy mentioned for Western Mediterranean dishes (e.g., socca/farinata, panissa, pannelle) and toward the gram flour for Indian dishes (e.g., pakoras).
The new Hayden Flour Mill is offering chickpea flour. I've seen it at the downtown Public Market and the Urban Grocery. Pane Bianco also offers their flour. I don't remember if I saw chickpea on their shelf last time I was in but certainly they'd be able to get it for you.