It would be a neat toy but I don't know what you would use it for.
An 8 gauge might make a super waterfowl gun, except IIRC it's illegal to shoot migratory birds with anything bigger than a 10 gauge.
At 15 or so lbs, the 8 gauge will be too heavy to be an upland game gun, and it wouldn't be well suited to trap, skeet, or sporting clays.
As for availability - I've never seen one for sale anywhere. I think you'd need to go custom maker and expect to pay $5000 - $10,000. but that's just a wild guess. I don't think you could adapt a 12 gauge action to 8 gauge because the firing pins on a 12 sxs are too close together to work for 8 gauge barrels. So the action would need to be built from scratch.