I've never seen conventional flap disks finer than 120, usually the next stage finer would be to use a scotchbrite type surface conditioning disk, loads of different suppliers for them, here's one http://www.abrasive-systems.co.uk/surface_conditioning_flap_discs.html
for best results though you will need a machine that runs at about 3000 rpm rather than a standard grinder at 11000 ish,
I use a 7" variable speed sander / polisher with good results, just a little heavy for prolonged use.