This air stone is not a ring. But is round. It is nice, and put out a ton of bubbles if you have a strong enough pump. Check it out...

Bubble Disc 3 inch - Aquar Airstones at Arcata Pet Supplies



Or even this one, I just found. I like the Disk myself... Bendable Bubble Wall 48 inches - Aquar Airstones at Arcata Pet Supplies