Cheapest place I found it with shipping was on www.Ebay.de Ebay Germany had a lot of them and they worked out cheaper than Uk counterparts. Guessing that it being manufactured in Germany helps.
Good luck with the talking. If you need an argument to help you convince the missus, tests show that it saves you about 13% fuel. They have it on their website in the tests for the Mustang GT. You would recoup that money very quickly if you drove a lot. Just saying