Shard of Beronath blueprint can drop by random mobs, can be sold by Vinelton in Ancient Grove, probably too by Hyram in Malmouth area and also some monster totems may drop it. Sadly Infiltrator don’t have other options for auto attack and is one of the strongest builds in the game.
Luckily you can do Shadow Strike focused build in the meantime, using Spectral Longswords from Steps of Torment. Like this: