Where do I download a source RPM from?
You haven't given much information, so the easiest thing to say is:
In general, what you should try is to find a download mirror for your distribution. For example, the Fedora 14 mirror list is at mirrors.fedoraproject.org/publiclist/Fedora/14/i386/.
Then navigate to find the SRPMs folder for the release you have, e.g. something like /fedora/linux/releases/14/Fedora/source/.
Other useful resources:
Usually most rpm-based distributions make the SRPMs available on their mirrors in the SRPMS directory. Using that you can manually download the src.rpm package from the HTTP or FTP server. Also, there is another way, using yumdownloader as described here.
Personally, more often I get srpms manually, but, depending on what do you want to do, your mileage may vary.