The Void V18-1000 is a popular alternative driver that works well in this horn (Void is Rog Mogale's company). I know Void have a US distributor for their range of cabinets so it may be possible to obtain this driver through them (Nathan Short?).
My small collective has four 1850 horns which we built and loaded with the V18-1000. We've been running them for three or four years now and have been pleased with the sound.