This is probably a stupid question, but even after unpacking all fpk files, I cannot seem to locate the building art used by Final Frontier and Next War. The rest of their art is in their folders, where I would expect it, but the buildings are missing. This is problematic, as I would like to filch some of that art for my own mod. Could someone point me in the right direction?

I dont know about buildings (are there any ?!) but the units used in FF are in the BTS units folder, not in the mod folder. Maybe same with the stuff you look for.

FF doesn't use buildings, just improvements, so that's the place to look.
The NW buildings are in the BtS/Art/Structures/Buildings folder like the rest of the new BtS building art.

Most helpful! I've managed to lay hands on all I need. Thank you very much, gentlemen.