Mmm, you found it just as I found it too ^.^

Yes, the little dead-end pocket areas on the maps with the Antling Diggers have 2 of the flies in them each. Sometimes there is another critter (elemental, bomb) in there as well.

If you find a nice spot with two dead-ends near each other you can run back-and-forth between them and that should be just enough to make them re-pop.

Drop rate is not bad, and they do drop the +1.

The locations listed elsewhere (near Bluefog, or norther thanalan) also DO have a fly. But just one or two. Not really worth going all the way out there for. Since copperbell mines has so many.