All grand charms that drop mid Nightmare onwards can potentially be skillers. There is no "special place" where charms will drop with increased chances of being a skiller. If you clear high density areas though (like the Secret Cow Level), more GCs will drop, and thus your likelyhood of stumbling on a skiller per unit of time increases.
The best (potential) skillers come from Baal/Nih/Diablo, becaue their Ilvl is high enough to spawn the 41+ HP Vita suffix.
Not sure where you're trying to say about Hephasto.
Best place to find skillers is Lower Kurast, Cows and I heard even Black Marsh is good. In Hell mode ofc. As for LK, run it in a public Baal game (Diabaal runs are even better as they give you the time to run LK and even mix in a Mephisto or other mf run to avoid risk of temp bans, while keeping the /player 4+ benefit)and open the superchests that spawn next to the campfire (theres chest in each of the huts to the north and northwest of it), sometimes there are 2 fires and thus 4 huts with chests. Also kill the uniques and champions there, LK is good for jools charms gems and runes.
If you mean skillers with +life, then best bet is to run the high bosses of the game (Baal Nihla and Dia) or get a Baal GC (one dropped from Baal or Nihla only) and reroll it.
It should specify Hephasto (first kill) and Hephasto. It does that with all other major bosses, so if it doesnt do it with him maybe he doesnt have a special quest drop. Never paid much attention to him in mfing or in ATMA.
The info displayed on ATMA is for Hephasto's drops, not for the Hellforge Quest reward (breaking Meph's soulstone with the hammer, as you put it). The HF quest drops a rune (a random one chosen in the El-Amn range in Normal, Sol-Um in Nightmare and Hel-Gul in Hell) and a few gems.
Including travical might be a good idea too. I tend to find many gc's when I run travincal. There's no gurantee for the gc's to be skillers, but the more gc's you find the more skillers you'll id I guess.
When looking for plain skillers, the best places to look are areas of level 50-54 (for instance NM Flayer Jungle), since any grand charm dropped here will have a slightly higher chance of spawning a skiller prefix. The word slightly should be emphasized though, since the only excluded prefixes have a low spawning frequency (Lucky alvl 55, freq 4 and Serrated alvl 67, freq 4).
Therefore (as mentioned) the best place to look is an area of level > 50, with high density of quickly killable monsters thus allowing for more GCs/time to drop.