Unless you are planning on having fish, unlikely in the size of pond you are talking about, you can just use tap water.

Hello Piplyn.
Did you see this on Gardener's World yesterday? I did and thought it was a lovely idea.
It did say use only rainwater, but that will be difficult if you do not have any way of collecting it. You could try speaking nicely to a gardening neighbour, but rain has been very short. Otherwise, I wouldn't bother boiling the water.
For the future you could plan a rain-collecting waterbutt to make the most of what runs off your roof. We have one by our greenhouse but not the most efficient so I'd like another one too.
Hope you enjoy your mini pond.

Tap water is Ok unless your water is very hard. If you leave it to stand for a few days the added chlorine the water companies add will have evaporated. We have used this for goldfish quite successfully even with moderately hard water.

Just a correction from me - Piplyn corrected me and the BBC programme we both saw this idea on was Springwatch last week, not Gardeners' World. Wanted to add this snippet of info in case anyone wants to track it down. It was a nice little piece of animation, good for interesting children I think.