TL;DR: (/way 69, 43 Azure Span - Lost Ruins) OR (/way 48.24, 25.05 Azure Span -Cobalt Assembly) if you need Cobalt Assembly rep.
Editing for newer information. The quest markers were bugged the first week so I'd simply posted the first place I found. A lot of places with Sundered Flame mobs aren't great for this quest, as many of the smaller dragonkin like taresk seem to not want to drop weapons. Cobalt Assembly can be good - especially if you still need rep or the world quest Cobalt Catastrophe is active, around (/way 48.24, 25.05 Azure Span -Cobalt Assembly). If you have no reason to go there, the area around (/way 69, 43 Azure Span - Lost Ruins) is the best place I've found so far, as everything but the spiders appear to drop the items. While there are other places with these mobs like what I listed below, or near Emberwatch in Ohn'aran Plains, they just don't have enough eligible mobs in the area to be worthwhile.
Original comment: While the marker in Thaldraszus appears to be incorrect, there is a way to complete this quest. Any Sundered Flame mob appears to be able to drop them - for example I fought mobs around
/way 12, 41 in Azure Span.