Got some Bosch retrofit flats online cheaper than normal ones can be had at Halfords. Passengers doesn't sit flush when parked but a year on and they are still as good as new.
I have always used wiperblades.co.uk for mine, really good service especially when I broke one installing it and they sent me a new set foc, the Bosch one are the best in my opinion.
I got the bosch ones from ecp. Cheap and work well but if I was doing it again I'd get bmw oem as the bosch do not sit flat on the windscreen passenger side. Looks pretty shoddy imo.