For those who have modified psp, I m sure you know what to do with "iso" and "cso" .
You can get the latest ones here for free;
http://www.pspmaniaonline.com
http://pspisoz.com
http://www.pspshare.org
Do not download through Torrents as there's too much risk of Virus and Spywares.
Use Mirrors, Direct link or Alternative Downloads from the websites which I mentioned above.
(Pls post below if you know any good websites like these.)