where can i buy heavy elven diagram if this thing is realy in the game ?

I cannot see why you would want it, as it's specs are far below others even in Act1.

The Magical, Rare and Epic items show up at some merchants periodically. and by chance. In Act1 I would keep checking back daily with the Mysterious Merchant and Blacksmith - but it is possible that it doesn't show up until a later Act / merchant..

I can't say I have ever seen that Diagram for sale, though it IS in the files.