Where we can get source code of scribus 1.5.3 svn.
please refer to
for getting and compiling the current scribus source code.
by default, you will not get 1.5.3svn but the current trunk/master. and it's normally the best thing to do.
if you just want the code that has been used for the 1.5.3 release, you can click on "source archives" on https://www.scribus.net/downloads/unstable-branch/