Solaris 8 was the first version to ever be available by download.
All earlier versions had to be purchased, then a full media kit was shipped to you.
Now, because even Sol-8 is so old, even that is no longer available for download.
[url=http://www.sun.com/service/eosl/solaris/solaris_vintage_eol_5.2005.xml][/u rl]
You only choice, at this point is to haunt some of the online auction web sites.
( Ebay is an example of such a web site )
Hope that someone will offer a media kit ...sometime.
By the way, what sort of hardware do you hope to install it onto ?
Solaris 2.5 will only run on ancient stuff.
If it's on SPARC, Solaris 2.5 doesn't function on anything with a PCI bus, for example.
You would need the last release of 2.5.1 to maybe run on old SPARC systems with PCI.