I bought wiper blades for a car some years ago and had the same problem. I went back to the shop and , lo and behold a chap appeared with a handful of bits and pieces, sorted out one that suited, fitted it and sent me on my way. The name of the shop ...... Halfords. No-one could have been more surprised than me.
If there is a large store near you it may be worth a try, after all, they must sell hundreds of the things every month and get used to fixing this sort of thing. You may have to buy yet another blade, but if it does the job a quick and cheap fix for an MOT.
Good luck !!!!!
Andy