This is my wife's plastic watering can. The original sprinkler head cracked and fell off. I replaced it with standard PVC parts from the local hardware store.
Why bother? Most watering cans I found on-line were metal with prices ranging from $25 to $50 each. I made my sprinkler head for about $3.50.
Step 1: Parts needed
The red numbers identify each part. Number 1 is a 1/2 inch male adapter. Number 2 is 2 x 1/2 inch bushing. Number 3 is a 2 inch slip cap. The slip cap has been drilled by me to make sprinkler holes for the water streams. The slip cap slides over the bushing and makes a fairly tight fit by itself. You could glue it, but leaving it without glue allows you to remove it later.