There is a common misconception that motherboards arrive with standoffs and screws. In most cases, this is not true.
With the PC Case, you will find screws and standoffs. Alternatively, if you are building a very customized PC with DIY enclosure of your own design, you can buy them from Amazon or Newegg. You can buy 10-20 of them for less than a few dollars, and a pack of 10-15 for even less.