Windows 10 Pro OEM from a reputable online retailer.
There is a lot of hate on OzBargain for such deals, but I think based on the reputation of the seller, that these keys are legitimate.
Jay has suggested SCDKeys for ~$25 https://www.scdkey.com/microsoft-windows-10-pro-oem-cd-key-g…
For the risk takers $5.36 (over 5,000 sold) https://www.ebay.com.au/itm/GENUINE-WINDOWS-10-PROFESSIONAL-… (this last one I received from eBay by email as a "recommended" product.
I paid $8 for my key.