I think
https://www.microsoft.com/en-us/software-download/windows7 should let you get the ISO.
The latest version.
EDIT: in virtualBox:
New > Pick a name, Type: Windows, Version: W7 64bit, Next.
select your RAM (stay within the green zone, 4-6GB should be enough, my green zone is ~12GB on 16GB RAM), Next.
Create a virtual hard disk now > Create.
Pick your type
(Chapter 5.2 Explains), I picked VDI, Next.
Either pick dynamic or Fixed size, I picked dynamic.
Select size of disk, to start off with I gave it 50GB, Create.
Now double click on the new machine on the left-hand side, it will have the name of the machine you chose on it. (You can also right-click and select start).
It will ask you for the boot ISO, select it and it will mount off of it and do the normal windows install.
If you cancel/"shutdown" the machine on that first boot, it will say no boot medium found when you boot it. At the top, select devices > optical drives > Choose image and select the iso.
EDIT2: Dynamic image you can increase the virtual drive's size if you need it, fixed can't but fixed is supposedly faster (I didn't notice a difference using either one of them).