Can't get 12s in screw on freewheel. Sunrace do a few nice one, sachs maillard used to but can't find em anywhere now. SJScycles have a number of regina ones but they are a bit touch and go with indexing, shimano ones now have ramps and stuff but start at 14 tooth (can you tell that I've been through the same thing recently...). The best I found was a 13 -28 sunrace from Jenson in the states - arrived 2 days after I ordered it, so not quite chainreaction cycles but nearly seeing as it had to travel several thousand miles.