There is no such thing unless you find a trustworthy source, whether locally or on Ebay, and buy used. These CPUs are many years out of production and any new stock out there for a 6th or 7th Gen i5 or i7 is going to start at about $180.00 and go up from there.
Used, you could do something like this.
https://www.ebay.com/itm/Intel-Core-i5-7600K-3-8GHz-Quad-Core/233458657278?epid=243929343&hash=item365b3867fe:g:etkAAOSwgslbHP6V&LH_BIN=1
But even that seems like a waste of money to me for a CPU (Assuming buying from North America rather than China, which I would advise you to 100% avoid because you are seriously rolling the dice if you do that) and platform that are many years old. If you can find a 6th Gen (Skylake) or 7th Gen (Kaby lake) i5 or i7 locally for a decent price, meaning less than ~150 bucks, it might be worth it. Otherwise, you'd be better off to simply sell that board to somebody who already has a compatible CPU and needs a motherboard, and there are plenty of them out there, and buy into a newer platform like 3000 series Ryzen which you can get into with just a board and CPU for around 320 dollars and will get better performance than you could ever get from that platform you have, even with a 7700k which is the highest possible CPU you could put in that board.