It sounds like you want to buy windows 10?
You could get it on amazon.com or amazon.co.uk if you are from there.
You can buy windows 10 in many online shops.
Be prepared that it cost around 100$ depending on what version you want to buy (home, pro, etc)
if you find a cheap stick with windows on it, it might be pirated software and even could include maleware.
In the early windows days they alsways put a disk with windows on it next to your pc box.
but i Think since windows 7 the installation cd is stored on your hard drive (and you need to burn it)
if you cant reach you hard drive anymore you could try to contact your computer manufacturer
i do not know the rules about linking in this forum, so i did not provided you a link to amazon :)
But i guess you can find it on your own there.
With kind regards,
Fil