Never mind necessity, a spending freeze is the mother of invention. I haven't found a really cheap watering can yet, so I decided to make my own.
I took a clean 2 litre (4 pint) milk bottle and cleaned it.
Then I used our cordless drill to drill a bunch of holes in the front right up by the neck:
And a couple of air holes in the back above the handle:
Fill with water, tip upside down and voila, watering can!