ya i've checked those ... dont seem to find SP 13 downloadable there
is it possible that as part of SP13 oracle has given only patches and no direct download and would these patches be applied on sp12 installation?
could get these patch details from forum
145531-01 - Sun Java System Web Server 6.1 SP13, sparc filebased patch 6.1 Oracle Solaris on SPARC (32-bit) (American English) - 128.9 MB
116648-25 - Web Server 6.1: Sun ONE Web Server 6.1 Patch WS61SP13 6.1 Oracle Solaris on SPARC (32-bit) (American English) - 25.9 MB