You can buy these in grits through 400 and in sizes from 1/8" diameter to several inches.
I don't use them in a power unit, the bands are mounted on a mandrel that is stuck in the end of a handle. Make you handles comfortable, it takes time to do the sanding. If you try a power unit it has be slow RPM or you won't get as smooth flutes as you can by hand.
I cut my flutes, then I have a wood handle that these fit on. I have one for each grit and I can go around an entire call, then pick up the next one and keep going until I am finished. I also cut my flutes so that they come out of the wood, not a dead stop leaving the end rounded it slopes up. Lot easier to sand them that way. For each grit I run the sanding band from top to bottom in a smooth sliding motion, kind of twisting it as I push it in the flute. An adjustable Panavise and holder helps or you can hold the call in your hand. Find a pic of BigB's checkering cradle that is better than trying to hold it.
Marvin