For aftermarket, see
Wisner's ; they makes excellent 5-rounders; they are marked with just a "W" on the bottom. They are available directly or from Brownell's.
If you are looking for a genuine Winchester, try
Out Back Gun Parts "Out Back Gun Parts, Inc.
2110 McLean Avenue
New Albany, IN 47150
Or
812-945-0480
Tuesday through Friday 10:00 AM to 5:00 PM Eastern Time"