instead of finer flap wheels, have you considered selling these:
http://www.progresstool.com/cat_redial.cfm
i have a plethera of these from the 1", 3" and a bunch of the 6" they are made mainly for jewelers i believe, but these things save so much time i will never touch another flapwheel. they are a bit more expensive, actually the 6" wheels are $25ish, but one 6" wheel will last me about 40-50 hours of sanding. i haven't used any of my 3" wheels yet, but the 1" wheels on a dremel stacked three high will do about 10-20 hours of detail work, depending on the edges of what you're working with.
as said before, i will never use another flapwheel because the benefit far outweighs the higher cost, esecially being able to go from about 1/8" wide to 1"+ just by stacking more brushes on top of each other