Here are the RISC OS 6 pages

http://select.riscos.com/RISCOS6/index.html

And it is now downloadable, but you have to pay a subscription fee to help further development to get it.

You can also get an open source version of RISC OS 5 here

http://www.riscosopen.org/content/

Be prepared, though. It's like installing Linux used to be before more noob friendly versions like Ubuntu started appearing.