I'm doing the succubus quest and I need to prove to Iorveth that Eli'yas is the murderer.

where do I find these bladeshards?

You find them examining the arm of the victim in the dwarven tomb ONLY if you have in your inventory the "Surgical Tools". You can buy them in Vergen from the mage apprendice in the market zone, they are under the category "quest items".

She also sells a book describing how to do an autopsy, but I don't think you need that in your inventory to find the bladeshards. The book just hints that you'll need the aforementioned surgical tools.

thanks!