There are many end users that are still using windows 7 professional and the disks may be available from your computer manufacturer. For example I still have my windows 7, 8 and 10 disks for factory reinstallation that I got from HP. HP business
recently informed me that for windows 10 that they are no longer selling DVDs but are now selling flash drives with the operating system. HP has moved from selling business computers with optical drives to selling computers without optical drives and
drives available as accessories or add ons.
I just checked my windows 10 iso bootable flash drive and file explorer displayed 28.5 GB free of 31.5 GB. So you will need a devoted flash drive (it will format whatever is on the drive to install the os) of at least 5 to 10 GB. The downloadable
windows 10 iso files size is likely to increase with time so the larger size is preferred for future downloads.
If you want to download and install Microsoft windows 10 it is available at this web site: https://www.microsoft.com/en-us/software-download/windows10
It is free if you had updated during the free upgrade period that ended July 2016. I don't know the cost today and you would have to check.
The windows 10 iso download to a flash drive can been handy for operating system reinstallations as a bootable source or for operating system troubleshooting/repair.
If you are interested in windows 7 only these links offer additional information:
http://www.zdnet.com/article/seven-perfectly-legal-ways-to-get-windows-7-cheap-or-even-free/
https://answers.microsoft.com/en-us/windows/wiki/windows_7-windows_install/clean-reinstall-windows-7/0ae269fd-167e-423b-94ef-26133f89e10c