According to 'Races of Faerun' (page 160) "...the dark elves carefully guard the secret of their venom, and it is exceedingly difficult to find it outside Drow relms and outposts." I'm not seeing it mentioned anywhere in the FR handbook. The only other place I can think to look is 'City of the Spider Queen.' Other than that, I think this is one of the few things Wizards left up to the DM. Good luck!
~Alloran