Some hosting providers will not provide a traditional SSL cert. This is especially true if you decide to use wild card SSL on your internal IIS server, while it is an issue to your web server hosting provider. In many cases a cert will be provided as a .cer and key file.
But IIS will not take .cer since it will be missing a private key. In this case easy solution is to convert .cer and key file to.pfx.
This site explain in detail how to do that https://www.sslmarket.com/ssl/how-to-create-an-pfx-file
But if you need a quick solution, you can easily use this online tool to perform that task and generate the pfx and install it on your IIS.