If there's a Starbucks nearby, you could get coffee grounds from them. They have a corporate policy of making them available for gardens. Other coffee shops may be willing to save them for you, as well, but Starbucks does it as a matter of policy.

Your grass areas will like you better for leaving it with the grass clippings. And its a lot easier and faster.

But, organic matter isn't just going to appear magically for the garden area. The _easiest_ is to just buy a load of compost and have it delivered ever so often. Of course, that costs. Option B is to find folks that are happy to get rid of their stuff. Horse barns, stables, lawn care places that catch grass, folks getting rid of leaves, the coffee houses as mentioned above. You can compost or not, depending on what you are getting and your personal preferences.

I'll mention just ONE very obvious source of organic matter, then link you to a good thread in another forum.

Collect and grind fall leaves. We chop them up with the mower.

Here is a link that might be useful: Organic amendments thread.

Any animal feed you can get at the local feed store is a great organic fertilizer. I just bought a bag of rabbit pellets (pure alfalfa). The protein and carbohydrates feed the soil microbes and they, in turn, fertilize for you. Corn meal is good, corn gluten meal is very good, soy meal is very good. All these are available in 50-pound bags for under $10. If you use something that is pelletized, you must moisten it when you apply or else the birds will carry it all away. All you have to do is moisten it, not wash it in. When the pellets get wet, they soften to the point of being mushy, but they still hold their shape. You'll see when you try it.

The application rate is 10-20 pounds per 1,000 square feet. In a planted garden I usually scatter one heaping handful under each plant on a monthly schedule.

You can make your own compost which is a great source of organic material for the garden.

In additon to free homemade compost, other amendments would be the animal feeds that dchall mentions.

(And even though you didn't ask, I feel obligated to suggest that you should reduce the size of your lawn... Living in Illinois, the prairie state, your lawn should be 0.25 acres mowed turf and 1.25 acres prairie grass :-) )