We get most of our belts from A&H ABrasives:
http://www.customsandingbelts.com/They have reasonable prices, good service, and they can make any size belt you need.
Their joining technique leaves a small "bump". I've ordered some belts finer than 120, but at those grits the bump becomes very noticeable, so I stick with 120 and coarser belts (which seems to be adequate as a good starting place for hand stoning/polishing).
Klingspor is another supplier people mention:
http://www.klingspor.com/They have a wider range of belts and joints. I've been meaning to call them and get their advice about the best joint & material for finer than 120-grit belts.
I use alumina belts and pay close attention to the part of the belt I'm using and move to an area of fresh grit after a few passes.
I have a couple of coarse-grit (50-36) belts in zirconia and ceramic since they are better at removing large amounts of material (ex. when you are putting the first edge on an unsharpened piece of steel, really changing the profile of a runner, or working with 3/8" and wider runners).
My "go to" range of grits for normal sharpening is pretty small, 80 and 120.
80 is only for
really bad runners; most of the time I use 120 (esp. on 3/16" runners) for touching up small nicks or minor profile changes.
