You can pick them up on eBay new for about 60 quid which isn't far off the cost of the OEM one which needs replacing every so often, so it makes sense to me.
If you get one, keep the old filter in a box in case you want to use the car when you decide to remove and clean the BMC filter. Personally, I like to leave them to dry at least overnight before refitting, though it's such a convoluted job to replace the air filter on these cars, you'd probably end up removing, cleaning and refitting before moving the car anyway...