A PSU doesn't need an adapter as such (at least it's not called an adapter). It just needs a mains cable with 3-pin mains-plug at one end, and a 3-pin female connector at the other end for the PSU power input.
Like this:
http://www.argos.co.uk/static/Product/partNumber/699380.htm
You don't get the cable with the PSU as many people don't need it if they are just replacing an old or dead unit, in which case a supplied cable would probably just end up in a drawer or, worse still, in the bin.
It would also add a few more pounds to the cost of the unit, and many buyers wouldn't tolerate that if they don't need the cable.