It’s recommended that the 24V AC transformer is rated 500mA or above. If you searched for ‘sprinkler transformer’ or ’24v ac transformer’ on Amazon, there should be some available choices. For example, this one on amazon.co.uk:
http://www.amazon.co.uk/Orbit-57056-European-Transformer/dp/B004RUHA5M/
As long as it’s AC transformer, it doesn’t matter which order you insert the wires — AC has no polarity.