Find people who will donate their land to conservation trusts, whether out of the goodness of their heart or a tax donation. As an owner, you can even keep the land with a conservation easement if you prefer (this is ideal if you're not near end of life, where you might want to continue to enjoy the land versus transfer it to a conservation steward org); you get the tax deduction and continue to own the land, but you cannot develop it. Buying it, imho, is a last resort, because of the legwork to put the funds together. This buy was $35M, for example (or rather, this announcement indicates that was the listing price; the transaction price might be lower, would have to look at property records to confirm).
(i work with a land conservation trust in the midwest)