You can download Windows 10 installation media directly from Microsoft here: www.microsoft.com/.../windows10
Or if you'd prefer to avoid a clean install of Windows just because you have to replace a hard drive, consider using a tool like Macrium Reflect Free to capture an image of your current hard drive to an external hard drive, then you can install the new internal drive, boot into Reflect's Rescue Media (which you create when you first launch Reflect), and restore the image onto that new drive. Or if you have a way to have both drives connected at the same time, e.g. a SATA to USB adapter (very inexpensive if you don't already have one), then you can run a clone job directly from the current drive to the new one, bypassing the intermediate step of storing an image backup file somewhere temporarily. You can even run that clone within Windows, although any changes you make after the job begins will NOT carry over.