Montblanc releases spare parts neither to end customers nor to independent workshops, so a sleeve for the feed cannot be obtained through the normal channels. And even with a part from a donor pen to hand, what is generally missing is the special silicone sealant with which the sleeve is bedded leak-tight into the ink reservoir. Without that sealant the pen leaks inside the barrel, so repairing it oneself leads nowhere.
The sensible route is the manufacturer's own service, which charges flat rates; the lowest service rate stood at 39 euros in 2005 and the work took a few weeks. As a precaution, the nib unit of a 146 should only ever be turned with light pressure and never with tools. If dried ink has seized it, a long soak of the section in water helps far better than forcing it round, in the course of which the collar of the sleeve breaks off.