Free: Contests & Raffles.
https://www.camofire.com/ has Footwear Fridays. Sometimes Cripsi, Salewa, Kenetreks pop up. Other than that guidefitter.com if you're a vet or 1st responder.
REI garage sale is where you will find the best deal on boots.. 9 times out of 10, they are already broken in and they can be 20% of the original price. I have some plastic Koflach's I bought for $50. And a number of other very high end backpacking boots over the years.