I think you're best off regularly checking eBay and the likes, though if you do know of any breakers yards I'd give them a go, alloys are very very rare now for Micras.
I think all K11s are 4x100 on the stud pattern though, and 13/14 inch wheels, so any old 4x100 wheels will fit on the Micra. Mazda Mx5s have some that don't look too far off the Micras stock wheels, and they are all 4x100 too and relatively cheap.