After speaking with Zainsubani and giving you the Meet Sul-Matuul quest, Caius suggests that you acquire supplies from Somutis Vunnis and Crulius Pontanian at fort Moonmoth. UESP says that you can get the items by asking about "essential supplies", but the option isn't appearing for me. Does anyone know why? It's not crucial to the quest, but being the completionist that I am, I'm curious as to why it's not showing up.

I think in the original game there are some missing Journal checks for the dialog variables.
The MWPatch Project fixes this glitch.

I have MWPP installed

Hmm,
you're not getting the Topic at all?
Must be missing a journal somewhere. :shrugs:

This should help: AddTopic "essential supplies"

Well that worked just fine. Weird that it didn't show up before.

This might seem stupid, but are you shure you are talking to the correct people? I believe one of them is in a corner room, like a temple with money on the alter. If you are in that room then I have no idea what to say then. Use the ~ command that Pluto suggests, see if that works then.

I think that the topic "essential supplies" will only appear if you clicked on it when Cassius first mentioned them.