There's a side mission you'll undertake to rescue some Cerberus scientists who have defected. This mission is titled "Arrae: Ex-Cerberus Scientists." I don't want to spoil it, but it's a side mission that can potentially involve one of your former Mass Effect 2 squadmates.
During one part of this mission, you'll be able to wander around their lab. The data about the poison is located in their lab. I believe it's on the second floor, in a corner away from the large plate glass windows. There's an NPC that you'll have to talk to in order to progress the story, and I believe you should search the lab thoroughly first, lest you miss this item.
There's a second opportunity, near the end of the mission, where the same NPC will ask you if you're ready to leave or not. I told this person I was ready immediately, but MBraedley confirms that if you want a second chance to check the lab, this is it.
Although I found it in the lab, the ME Wikia states that if you miss it, you can buy it on the Citadel afterwards. The credit reward for the mission is the same as the cost of the item, so if you missed it, the only reward is +5 Reputation, 30 XP, and a small War Asset boost from the general you cure.