I can’t drop this wand, i try mefisto, diablo, baal … nothing, i trying 2 month, my god, can you help me? where i can find this wand, i’ve no idea what can i do more … 
Arm of King Leoric is an Exceptional Tier Unique based on a Tomb Wand. It resides in Treasure Class 45. It has a Quality Level of 44 so basically any Champion/Unique pack in Throne of Destruction on Normal Difficulty will be the first instance on where it can drop since the Area Level is 43(what’s needed for Tomb Wand to drop). The next instance on where it can drop is Act 1 Catacombs on Nightmare(specifically Level 3 and 4). After that, anything in Act 2 and beyond has the chance to drop it. Whether it will drop is purely up to RNG so when you’re hunting a specific item, the chances of it dropping are purely luck.
I’m in the same boat, but with death’s web. I’ve found 2 Arms of King Leorics, but no death webs so far lol.
Few months late but Death’s Web is one of the rarest items in the game. Getting an ideal one with +2 Necro/+2 Poison&Bone and -50 Enemy Poison Resistance is considered a Holy Grail find. Maybe not as much as a 20/20/10 Anni but darn close. A perfect one sold on Non-Ladder for ~8 Jah(source: Traderie). For Ladder, a 2/2/-47 went for Lo + Um + 40 PGems while a 2/1/49(not counting a 5/5 Facet) went for a Ber on Non-Ladder so the price can vary widely.