Go to Walmart and go into their deli or bakery section (most have one or the other) and ask them if they have any empty egg crates. Generally they will give you as many as you want since they go through so many eggs and toss them anyways. I got over fifty in one trip once, all free.
Not bad prices and they sell the 30 egg or 50 egg flats in packs of 10 for around $3-4. With shipping it's 12 bucks. Enasco.com is pretty cheap, but I don't need that much. You can get them for free at certain places like many have said, but right now I don't have any places around to ask.