Restore IMac 8.1 OS X El Capitan

I've just got an old iMac that I need to install an OS on. It doesn't have internet recovery. The iMac is the only apple product I have so I've tried several times making a bootable USB with windows and no luck. The USB stick just does not show up in the options when I press options on boot. It does show up in the disk utility but when I press info it sais it is not bootable even though it's in OS X Extended and GUID Partition Map. When the computer turns on it goes straight to the OS X utilities menu with options for disk utility etc. There's also an option to reinstall OS X 10.11 but I always get the error "This item is temporarily unavailable, please try again later". This is a second hand device so I can't get the username or password it was initially set up with I've tried changing the time and date as I saw on various forums but still no luck.

Model number is A1225

Is there any way to install the OS from here? I've never used and apple product before so I didn't have El Capitan downloaded to my account, is that why I get the error?

Any help is greatly appreciated, thanks