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.
1 - http://getintopc.com/softwares/operating-systems/
Windows (7, 8, and 10):
1 - https://www.microsoft.com/en-us/software-download/windows10
If you guys need to get programs loaded onto your VM fast this is a very good tool to use:
Ninite - https://ninite.com/