Dustbin, that's what I'm using.
You don't necessarily need a tap - it's quicker to dip a watering can into the top than it is to wait while it fills from a tap, so any large waterproof container will do. You do need a lid, to keep out dead leaves and mozzies.

And if you're looking to store a lot of water, consider those 1m square water cubes that they use on building sites in remote areas - no idea how much they cost, but they might cost less than the equivalent volume of water butt.