I presume you are referring to an optical drive to load the Win10 DVD? If so...
They are relatively cheap if you want to install one in the system permanently.
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&IsNodeId=1&N=100007589%20600003509%20600003707
You will need an extra SATA data cable and one of your power supply's unused SATA power connectors to complete the hookup.
If you don't think you'll use the optical drive much, it might be easier to simply buy a USB drive. Plug it in, install the OS with it, and unplug it until needed later.
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&IsNodeId=1&N=100007595%20600003515
Another option you can take, is to download the Win10 .ISO file of the version of Win you have. Then use the media creation tool to convert it to a flash drive. That way you won't need a optical drive at all. You can get that here:
https://www.microsoft.com/en-us/software-download/windows10
The license (product key) you have for the DVD will be used to activate Win10 when asked.