Solaris 8 was never available as a download.
It was only offered as commercially presses media kits.
You will need to haunt your favorite online auction web site (for example, eBay).
Solaris kits become available at such sites occasionally.
As I type this, there do happen to be a few full kits offered. Most are really old releases, but they are Solaris 8.
So even if it was already acquired by Oracle it was not place on OTN downloads?
Oracle acquired Sun in 2009. Sun Microsystems had already put Solaris 8 to end-of-life (EOL) status in 2007.
I have observed that Oracle retires all products in what can be described as a very assertive or aggressive manner, so as to only present current products to customers and potential customers. The company never goes backward to "what once was".
As for Solaris 8, they would have had to assign a person or a department to go through what essentially is a remanufacturing/re-marketing/re-packaging process to create ISO's from existing (but no longer sold) media, validate that the electronic files were viable, and place them onto a delivery channel. That would have been a counter-productive exercise.