Okay, I found my kernel source and specified that option when I ran the installer. It still said that it can't find the "NVIDIA kernel source."
So I did a little experiment: I ran it again, specifying the same option, but changed the source directory to one that does not contain the kernel source. This time it complained that it couldn't find kernel.h!
I changed it back to what it was, and it no longer complained that it couldn't find kernel.h, but complained again that it can't find the NVIDIA kernal source.
So it is finding the Linux kernel source that it needs. What it can't find is the NVIDIA kernel source. Whatever that is. And wherever it is.
On the plus side, I'm learning more about how to query packages and where to find stuff on my system than...well, I ever really wanted to know, but I'm glad I know it now.