The best source of rainwater is from the roof of a greenhouse, if you have one, or from the roof of a house. Green houses usually have rudimentary gutters so you could block one end with plasticine, silicone sealant or similar and collect the water from the other end. Direct the water into the butt or barrel with a scrap section of PVC guttering or similar. You can tie this where it makes contact with the gutter of the greenhouse with light wire, string etc to stop it moving around.
Water can also be sourced from the downpipe on a house. If you have plastic downpipes this is relatively easy. It is possible to buy a device called a downpipe diverter which attaches to the downpipe and performs this function.
See link: http://www.reuk.co.uk/Rainwater-Diverter.htm
All sorts of containers can be used for collecting water. Oil barrels if you can get them unused are ideal, alternatively you may be able to get old plastic barrels which you could clean out with detergent and hot water. You can buy butts at garden centres, stores like B&Q and Homebase etc. Another option is an IPC container. These hold 1000 litres and have a valve at the lower end from which water could be released as needed. One of these could be mounted on some blocks to keep it up off the ground so that a watering could be placed under the valve.