IIRC, I got them from Cedric in a prior play-through, but Cedric has disappeard . Is there a secondary source of either the bombs, or the fromula? Can't destroy the nests without 'em.

According to the wiki, he is the only source during Chapter I. How far are you into the story?

It's not unusual for Cedric to wander around Lobinden. Check in the map for the orange crafsman icon, especially if there's one moving. You can hover the mouse over the icon to see if it corresponds to Cedric. If you have already done that, try to meditate and let some time pass, then look for him at another time of the day.

I have to agree with CC. In fact I've found Cedric inside two different houses bordering the town/village square; as well as listening to the storyteller at night. I've found him in the forest just outside of town as well.

Also, don't forget about your saved games. You can always load a version of your game when Cedric was easily found.