Getting source rpm

Using the asterisk.org package repository (wiki.asterisk.org/wiki/display/ … eLinux5%29), I installed asterisk-dahdi version 1.8.15.1. Is there a way I can also get the source rpm for that package? Doing something like

does not work (No source RPM found for asterisk-dahdi-1.8.15.1-1_centos5.x86_64)