How can I find skillers/what is the first, or best place to find skillers?
Skillers, or Grand Charms with +to a character skill tab, require an item level of at least 50 in order to spawn. The first area in which all dropped grand charms have an item level of 50+ is the nightmare Great Marsh.
All "levels" are explained in much greater detail in this post.
http://www.battle.net/for...52349&p=1&tmp=1#new152349
In order to maximize your chances of finding a skiller, you want the item level to be as close to 50 as possible. Why is that? Because when affixes are selected, they are chosen from all available affixes. Having a GC with an ilevel of exactly 50 is the best in this regard because they do have the possibility of getting a skiller, while the possibilities of other affixes are being kept to a minumum.
However, there is an alternate approach. Items dropped from Hell Baal have an item level of 99, and items dropped from Hell Nihlathak have an item level of 95. Grand charms dropped from these two bosses can spawn with very, very high life, namely the 41-45 to life vita suffix. Grand charms that both have +to a skill tab and high life can trade for astronomical values. However, due to the high item level, most any affix can spawn. Your chances of getting a skiller from an ilvl 95+ Grand Charms are very slim, and the chances of getting that Grand Charm to spawn with the desired 40+ life are even slimmer. However, the time and effort needed to aquire these are worth it to some.
There is a way to significantly shorten the amount of time needed to find a skiller compared to just hunting the most likely drop zones over and over. This is referred to as "rerolling" or simply "rolling". This is done by placing a Grand Charm and three perfect gems in the Horadric cube, and transmuting. This will reroll the mods on the charm, but preserving the item level. This means that if you have a Grand Charm with that perfect ilevel of 50, but it has useless mods, you can reroll it into a useful skiller. In effect, it drops the item again at the cost of three perfect gems. Be prepared to spend a lot of perfect gems, but it can be worth it.
Note that this formula works on all magical items, but it is typically used the most on Grand Charms due to the high usefulness of blue Charms compared to the usefulness of blue equippable items.
Footnote: I must admit that I slightly lied about the first place you can get a skiller… the first place would be normal Baal. The items he drops have an ilevel of 60, more than enough to get a skiller. But the nightmare Great Marsh is the first area in which every monster has the possibility of dropping a skiller.
A common misconception is that MF increases the chance of getting skillers. That is simply not true. This lies in how MF works, and I direct you to Gta's Guide to all that is items again for explanation.
Good areas to hunt for skillers and rerollable Grand Charms include:
The nightmare Great Marsh – the first place they can drop, the area level is 50.
The nightmare Flayer Jungle – run more often than the Great Marsh due to the number of Flayers, which can all drop ilevel 50 Grand Charms.
Nightmare Cows – there is a very large number of easily killable cows here, and the charms they drop have a decent chance of becoming a skiller.
Nightmare City of the Damned – there is a lot of Wraith-class monsters here.
Hell Baal/Hell Nihlathak – both have the capability to drop skillers with 41-45 to life.
ADDED: It seems that hell Diablo can drop high life skillers as well, although he is an inconvenient boss to run.