How to purchase Windows for Boot Camp?
If Windows 8 has been released in your country, you should be able to find a reseller with the System Builder version available as a DVD. Local system builders is a pretty big business in most countries so just look around for someone offering custom built Windows systems.  A quick Google search on "create a DVD ISO on OS X" shows lots of how-to articles for that step.

You can either use an external DVD reader (maybe $30) or get somebody with a DVD to create the ISO for you.