You can buy QuickBooks Desktop (QBDT) program from retailers or third-party sellers. They're also the ones who will provide you the activation code so you can start using the software. If in case the code given won't work, I suggest reaching out to our Customer Care team to help you activate your product or to purchase a new one. You can also ask them about the legalities of using the program.
I've also attached this helpful article for your reference as you learn the basics of QBDT: Start Here with QuickBooks Desktop. It includes steps on how to download, install, and register the product.
Furthermore, QBDT has a set of guides and tutorials to help you with your navigation. This includes training, support options, and user guides so you'll be more familiar with the program.
Get back to me here if you have additional concerns about purchasing the QuickBooks Desktop version. I want to ensure your success. Keep safe.
I noticed in a previous post, you mentioned you bought QBD 2018 UK version. Why do you need to buy QBD 2013? Mind you, QBD US version and UK version are not compatible with each other. You have to purchase a 3rd party service to convert data files between country versions.