I read with interest the story of the Mitsubishi ASX owner unable to get replacement tyres for his new car, as this has just happened to me with my new Renault Laguna. I needed a tyre and was told the particular Michelin I needed was superseded by a new one. The new tyre was available, but the old one that was fitted to all new Lagunas in Australia had to be imported and could take months. When I called the Renault dealer and Renault Customer Service I got the same response as you did namely that the tyres were available and there shouldn't be an issue with getting a replacement. The reality is they are not available and I had to buy two new tyres. At the end of the day when all the spin, fluff and froth from Renault Customer Service is over you are late for work, you are still at the counter of Bob Jane T-mart and still can't get a tyre out of Michelin. The dealers did their best to assist, but Renault Customer Service was totally useless and simply fobbed me off and wished I would go away.
Tyre makers typically replace their tyres every three years or so and you can run into problems like you have with your Laguna. You could buy one tyre of the new type to replace the punctured tyre, and legally run it on the rear. You could also fit the new tyre to the spare and fit the original spare to the car.