Visit an exhaust and tyre place that does the Power flow exhausts, they make them on site and will be able to make you one up, pick up a nackered cat from a scrappy and they will weld the ends onto a piece of pipe and you'll be able to swap back at MOT time.

They may have some suitable fittings at the garage if you can't get a cheap nackered cat.

Or give people like Magnex etc a buzz or a supplier.