STORMIN NORMAN’S RACING, Billet clutch baskets , Basket wear inserts kit, includes machining (select models) call Norm at 724-722-3630
He has a website also, but I can't remember what the URL is. You can also go with a Hinson billet basket.
The main thing you have to watch if you file them down by hand is to try to file each finger as evenly as possible, to remove the same amount of metal from each finger. If you file some more than others, the clutch tangs will not touch at all points, causing rapid clutch wear, plate breakage, and failure.