So you have some options:
1. Try your luck with grey market like Kinguin.net
Be aware these keys are often improperly resold or used in violation of their original license, and can not work or terminate without warning. You could easily get screwed, with little recourse... their lack of liability in it is one of the reason they're so cheap.
2. Buy a copy. Microsoft sells them direct, most retailers associated with electronics will have one for sale too.
3. Windows 7 and 8 keys still activate Win10 as of writing this. It's not 100% above board, but it seems like it works. I did this just recently with a new build.
4. Microsoft Education. If you're associated with a University or sometimes highschool, tradeschools, etc. they often have support for a program called Microsoft imagine. As of last glance, Win10 was available for free for personal use... for a single copy, anyhow.
5. Some employers or the above will offer a "home use program" which lets you get things like Office or windows super cheap, but it's managed by your employer so you'd have to ask them how that works.
If a key you obtain through these vectors gives you trouble but you're sure you're in the legal right, try using their phone activation. My experience has been it's more liberal about the rules than its other vectors. In an elevated command prompt on the new system, type slui.exe, 4
Or at least, it let me activate a mandarin-only version outside of China for a chinese exchange student customer who's PC I repaired.