Keep checking gunbroker and auctionarms. They come up now and again- but pricey. Shipping is also steep.
There is a modern smokeless plastic cased shell CALLED an 8, but it will be headstamped INDUSTRIAL- flat fronted lead slug, used to tap steel furnaces by shooting out a clay plug in the furnace. Some folks claim they are for blasting out slag.
The HUNTING shot shells were all black powder loads (one ultra exotic exception in modern Euro weaponry). Since Federal game laws cut off at 10 g, do not know of anyone loading 8s or 4s. AND it can be very easy to wander across the line into the category of Destructive Devices if you cannot show a sporting use for a firearm.
I have a couple of 8s, and a single 4 g (not for sale) in collection.
The last 3 shells on lower right are 2 8s and a 4. For scale, the shells above them are 12s. Yes, that is a .50 BMG to the right of the 4 g.
BTW, do not think a 6 g shell was ever marketed. There were 6 g shotguns, but they were muzzle loader Punt guns.