But I don't know which items provide the Fulgur substance, and I've scoured the journal trying to find out. All I get is a blackened symbol in alchemy screen and no description of what plants/monster parts that may contain Fulgur. I do have Ostmurk already.

Easiest source to farm is Endrega Embryos, but there are a few other sources:

http://witcher.wikia.com/wiki/Fulgur

Ostmurk contains Fulgur, so you should not need to seek any other. But you may have to add the Ostmurk explicitly.

Select Fulgur from the alchemy substances (top right) and see if your Ostmurk pops up.

If it doesn't pop up, see if it's one of the ingredients already selected.

Thanks guys. I tend to go to the cave with Ostmurk before that particular quest is triggered after meeting Sile, and every time, it says I need Fulgur despite having the Ostmurk handy. I'll visit the cave after I meet her next time.