For Windows: you will need a Microsoft or Digital River ISO, if you are getting it online. If you do not have an image on hand, it is generally considered safer to buy an OEM DVD without a product key. Unless you can find other official means to get the ISO, I do not recommend using an ISO from the Internet.

Non Microsoft OSes: The problem with this is I do not know what you want to install. If I know how to find it I may be able to help you, but I cannot guarantee that I know for sure and can help you find the ISO you need. As such, you are probably on your own.