You need about 3 cubic yards. That would be a lot of bags.

Are there any horse racetracks nearby? Sometimes around a racetrack you might find a composting facility.

There is one in Arlington Heights. I'll look around. Is there truth that a composted horse manure will have tons of weed seeds?

Any horse farm would probably be thrilled to have you haul away some manure, and they likely wouldn't charge you anything... In fact, the farm where I keep my horse pays to have it hauled away in a dumpster every week!
If you can find a truck, someone would probably love to load you up & watch you drive away! :)
If the farm uses late-cut, seedy hay, you could find a fair amount of seeds. However, horse manure composts REALLY hot when left in an undisturbed pile, so they're rarely a problem.