I have missed something, or the yellow traitstones do not drop in the Underworld? Any kind of yellow traitstones.
Try the warrens. For some reason there are (or were) loads of arcane lights (yellow and green) amongst your rewards.
For some reason yellow traitstones do not want to drop in delves.
Arcane Light Traitstones are awarded in the Warrens boss room. The amount you receive depends on your room modifier. At the moment this is the only delve room that awards arcane traitstones.
All runics can be found in delves except yellows and reds which is why I have hundreds of every color but none of those.