Ive saw some good unique wand on internet but what the most efficient way to get one of those ? (i speak about leoric arm in my case)
Get some magic find and kill bosses till you get bored or get lucky.
I think it can drop from nightmare act 2 or 3 and up.
there is no way to craft those to increase the probability to get it with time ? or with charsi ?
Thankfully no. Thats not Diablo 3 
The only way to get uniques is trading, gambling or finding them. There is some crafting in this game but it is different from what other games have done and optimal crafting is a pain.
There is some target farming in D2, but dropping ‘the one’ item you want might be very difficult. You can check drop rates of bosses for a specific item and farm them. I dropped that wand but didn’t drop the gear I want rng and farm or trade.
Just swinging by to wish you the best of luck, Tenebros. Hopefully you can get your hands on the arm you’re after soon enough!
I will tell you what I would do.
Get to level 70s or so. Go to act1 vendors and try to find +2 BS wand. Get to larzuk make soc it you will get +2 and make White DolIo
Gg +7 BS wand