I doubt that you'll be interested in buying "Agua Bendita" in Copacabana, but who knows?
In that case, you can bring your own bottle as water is free. You can also buy bottled water in the church with a label saying "holy water from Copacabana" but it will cost you a few soles... easier to carry though and much better as a present!
We were walking along the trail when I spotted a llama right on the path. I snapped a photo and it scampered off to join its comrades overlooking the lake. I snapped another timely photo of them with the lake in the background and started back on the trail towards the port. I heard this kid yelling to "pagame, pagame," evidently for the "use" of his llamas as photo subjects. It would be fair enough if he stood there and asked for money but to ask for it after the fact seemed a bit presumptuous. I just kept walking as "no comprendo espanol." ;>