Hello Revan,
I have been spearheading the Netherdeep content of late (ruidium page etc.), and I am running CotN, too. As far as the missions go and the level of your awareness regarding ruidium, the following shouldn't contain any plane-shattering spoilers.
TL;DR: Ank'Harel.
The following are the circumstances I believe would be necessary due to ruidium's extreme rarity to have been exposed enough for mild to acute symptoms. Depending on your DM, the following could contain a few unwanted spoilers.
Ruidium is extremely rare in the material plane beyond its point of origin. It is isolated to Ank'Harel. One faction—the Consortium of the Vermilion Dream, a small religious group that venerates Ruidus—is primarily responsible for taking a sample from the archaeologists, called the Allegiance of Allsight, who discovered ruidium. The only group that possesses (a few) items containing ruidium is the Allegiance. The Consortium and the Allegiance have taken the sample and items, respectively, into the main city of Ank'Harel.
If your character already knows about or has been exposed to ruidium (mild symptoms only take a few seconds of exposure), then they must have had sufficient contact with the Allegiance, specifically member(s) who possesses such an item or have access to samples. Due to the clandestine nature of the Consortium, your character would likely have to be an established member to have been exposed to the ruidium sample that they possess.
Hopefully, this is helpful. Do keep us in the loop (perhaps through blog posts) about your experience in your CotN campaign with ruidium etc. Feel free to continue to reach out with further questions.
Best of luck on your adventure!
