A lot of schools and universities have programs where you can get an OS for like $20, so if you yourself or someone you know has access to one of those programs that'd be a good option. Teachers often have access as well.
Otherwise there aren't many options, there are some semi-shady websites that sell relatively cheap Windows keys but I wouldn't trust them. You can also get an OEM version, but that'll save you $20 at the most and you can use it with only one motherboard.