In San Diego

Just dig it out of someone's yard that you don't like.

Try your county recycling program. Here is Maryland there is a county transfer station where people and businesses drop off yard waste and the county recycles it.
You may need to borrow or rent a pickup truck to haul it home though.

If you just want "fill" dirt - which isn't necessarily the best dirt for planting - and may be rock, check here:
http://www.acehauling.com/filldirt.html

Try www.craigslist.com there's always something free in there.

Sometimes a construction site will deliver you some for a gas fee. Thats what I did. I got 20 dump trucks full for $20 each load. Just ask, you never know until you do. Or if you have a truck and don't want a lot they will use a back hoe or a bobcat (if they have one )and load it on your truck no charge.
Of course people in Missouri do things like that! I'm not sure about other states.
If you need a lot like I did the expense
will come in leveling it. Or you can rent a bobcat or grader and DIY!