I'm with Mardon too - partially for this reason.
From memory you arn't allowed more than two people per toy.
I have two standard Doughnuts - one a Jobe Thrasher (approx £70) and One a Sevylor not sure of model and picked it up second hand so not sure of new cost. The Jobe is alot better quality made than the Sevylor but they may not be comparable models.
Ropes you will need a tow line approx £10 and a bridle for the boat unless you have a tow eye / post. A suitable boyancy aid (if you dont have one already) is a must, as you will certainly get wet enough to fire an auto lifejacket.
Have a hunt around but http://www.roho.co.uk/
are good for other stuff diving etc but I've never bought toys from them.