Sourcing organic material for soil amendment

We have 3 acres of largely untouched land since it was cleared for the home over 30 years ago.

The soil on the back half of the property is very rich from foliage breaking down in it from the 90 year old oak trees and young dogwoods. It’s rich enough that the chickens can easily turn it over and dust bath in it and it’s black as coal.

The front half, on the other hand, has just been an open field with no exposure to anything other than draining water and sunlight. It’s a lot of grass and the soil is hard, clay, and not fit to grow in.

We want to til it under and throw as much organic material on it as we possibly can without breaking the bank.

I know about the lasagna method, with cardboard, leaves, untreated manure, logs, etc., but we simply don’t have access to any materials at the scale we need without paying a hefty sum or visiting a lot of stores for their cardboard - that is if they’ll give it to us.

How do you guys source large quantities of this stuff? Just ask farmers for their manure and buy cheap hardwood mulch by the cubic yard?