On Wed, 26 Sep 2018 14:02:37 -0000 (UTC),
NotReal , in

>  pi@raspberrypi:~/temp $ apt-get source sunclock
>  Reading package lists... Done
>  E: Unable to find a source package for sunclock

Did you update the package list?

sudo apt-get update

Once you do that, then you can do the apt-get source sunclock and get
something like this:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Need to get 1,585 kB of source archives.
Get:1 http://archive.raspbian.org/raspbian/ jessie/main sunclock 3.57-3 (dsc)
[1,208 B]
Get:2 http://archive.raspbian.org/raspbian/ jessie/main sunclock 3.57-3 (tar)
[1,571 kB]
Get:3 http://archive.raspbian.org/raspbian/ jessie/main sunclock 3.57-3 (diff)
[13.1 kB]

Then you should see 13 patches being applied, and ending up with a
directory called sunclock-3.57. From there, you can make your changes,
then build your own .deb package, and then "sudo dpkg -i
sunclock(stuff).deb" and install in the usual way.

> so I am probably not doing something right.  I would not think there
> would be so few sources available for that many binaries.  ???

That's because some of those source packages will generate many binary
packages. For instance, the source package gcc-8 generates 190 binary
packages.

Take a look:
https://packages.debian.org/search?searchon=sourcenames&keywords=gcc

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