One of the Drow's quarters in Velkynelve (the outpost where the PCs are help prisoner) would probably be the best place if you haven't left/dare to go back to the outpost.
(Spoilers as to who's quarters and some of the treasure within)
... while another purse is llvara's spare spell component pouch. Additionally, the chest contains any valuables once held by the characters and NPCS, including any spellbooks, components, focuses and magic items lost to the adventurers.
Depending on the makeup of the rest of your party, the monk could take and use one of these treasures in particular or if the character previously had the petrified eye of a newt on them before they got captured then that's a bonus.
Otherwise the campaign book specifically tells the DM that:
Spellcasters might be without material components for their spells. They can acquire component pouches and spellcasting focuses from defeated enemy spellcasters, settlements, and traders, or they can craft such items during their downtime activities while travelling. (pg.21)
So in short the best way to obtain the petrified eye of a newt is probably just to ask the DM for one. Depending on how hardcore of a game your DM runs, your friend might have to work for it but he may not be without one for too long.
Also, regarding how the newt's eye is petrified and whether it still counts as a material component; that's left entirely up to the DM. There are no rules that cover the specifics of how a material component is "made". The only thing the rules care about is that you have said material component.
This is quite a hard question, so I will do my best to summarize what I've found on Orcus and Demogorgon. Also, the 4E Book of Vile Darkness has a lot of information about demons lords.
Orcus
- Lieutenants: Vampires, liches, necromancers, cult priests
- Minions: Undead (ghouls, shadows), cultists
- Races: Not relevant for undeads, humans, orcs, half-orcs, ogres and giants
This may be the easiest one, because of all the detailed description about him. From the MM p. 52:
Orcus is worshiped by the undead and by living creatures that channel the power of undeath.
Which makes the choice for a lieutenant relatively easy. On multiple wikis (like here or here), liches and vampires are mentioned as his most powerful worshipers. In the 4E published adventure Keep on the Shadowfell, a human priest of the Cult of Orcus tries to open a portal to the Shadowfell. Also, from the MM p. 53:
Orcus, the Prince of Undeath, has the power to transform manes into undead monsters, most often ghouls and shadows.
We can assume he does that to make them serve him as minions. From the Book of Vile Darkness p. 138:
In particular, orcs, half-orcs, ogres and giants revere Orcus, as do a large number of corrupt and despicable humans.
Another quote from the BoVD p. 138:
Intelligent undead never willingly serve Orcus (they are more likely to venerate the deities Vecna or Erythnul). However, many vampires, liches, and other undead creatures are forced into his service by dark pacts or compelling magic.
Demogorgon
- Lieutenants: Severik the balor (mentioned in the BoVD p. 129)
- Minions: hezrous, aboleths, scrags, skum, various ichytoid beasts (BoVD p. 125)
- Races: tiefling, lizardfolk, half-elf (examples from the BoVD), kuo-toa (Out of the Abyss), Derro (Out of the Abyss). Mostly any kind of creature that has reached a pretty high level of madness.
Not many quotes for the Prince of Demons, but you will find more details in the BoVD.
Other demon lords
Well, when I started writing this answer, I thought I could gather information from websites, books and the like, and summarize my findings here. It turns out, the Book of Vile Darkness from 4E is really the best source of information about the demon lords, so you should probably buy it for the other ones.
Best Answer
MINOR/MEDIUM SPOILERS AHEAD!
First of all, the rules and reasons for the Drow Pursuit are very thin. Are they really supposed to chase some escaped slaves through the Underdark, when the demons are rampaging and their own city lies in ruins? On the other hand, the Drow Pursuit is a big theme for the "first half" of the campaign and a good tool for the DM to keep their players going towards the surface.
I started my campaign very by the book and have been keeping track of the pursuit level ever since (my group has been in Gracklstugh, Sloobludop and now they're on their way to Neverlight Grove and Blingdenstone). After leaving Sloobludop, I faced the same issue about the drow - why are the still after the PCs and if so, how do they keep track of them?
I offer my solutions for this dilemma:
So there's a lenghty answer that hopefully gives some ideas from a fellow DM. Perhaps all this can be summarized as "Use the Drow Pursuit to chase players around the Underdark for the first half and juice up their backstory a bit, as the book doesn't offer a straightforward or a sensible reason".