Sorry, Steve, the longest they stock is 10 foot - I tried there last week and got the sales guy to check.
I have located a supplier called Poles Apart, who do make a 12 foot square ridge pole, and after emailing them, they have quoted about $55 for the pole including a courier delivery... which is almost as much as the tarp itself! Might have to re-think this one... I don't want to spend more for the tarp (poles, pegs, ropes, etc) than I did for our whole tent.
I'm thinking the fact that they are square may be a contributing factor of the cost, because I don't think the round ones are that expensive - e.g. the 10 foot ones at Aussie were about $15!