I get asked this question sometimes, I mostly have any ISO files stored locally on my external drive for anytime that I might need them in the future but here are some sites that I use to download an ISO file when I need it.

3rd Party:

1 - http://getintopc.com/softwares/operating-systems/

Windows (7, 8, and 10):

1 - https://www.microsoft.com/en-us/software-download/windows10

2 - https://www.microsoft.com/en-us/software-download/windows8ISO

3 - https://www.microsoft.com/en-us/software-download/windows7

If you guys need to get programs loaded onto your VM fast this is a very good tool to use:

Ninite - https://ninite.com/