See my album for some photos of the OEM horns and location. As quickdtoo says, they are just behind the grill but there is not much space in there. I imagine you are asking as you want to replace them with something more intimidating than a flatulent duck.
Getting in there is pretty straightforward. Remove the large plastic shroud by pressing in the center part of the neat clips. Undo a bolt at each end of the metal cross support and jiggle it free. The horns are attached to this.
I went to ACE hardware and picked up a piece of alum angle and drilled this to run from the OEM horn brackets and then drilled this near the center to mount the horns. Used angle as I figured 1/8" flat bar would bounce the entire time and the angle won't. If using snail shell style horn, be sure to angle the mouths down slightly else I am sure rain would get in there as they are partially exposed behind the grill.
The OEM connectors fitted the Wolo Full Blast horns (one hi, one lo tone). Much better than the Toyota ones, but not super loud. Just sounds more like a horn. quickdtoo used some Freeway Blasters(?) - these may be louder, maybe not. Do a search on his other posts for more info.
Including the 20 mins to run to Ace, probably took 90 mins, but I was taking my time.
Hope that helps.
John