What's the cheapest way to buy cement in bulk?

So we’re pricing out what it would take to build ourselves a home. Because we’re doing it on the cheap, we’re providing our own labor and looking for ways to get the materials for, hopefully, less than retail price.

There’s just gotta be a better way to buy enough concrete (or Portland cement and fill equivalent) for a slab foundation than just going to our local Big Box home improvement handyman retail store (like Lowe’s or Home Depot) and buying individual bags of cement, and hoping for a case discount at the most.

Do you have experience with this, or useful ideas? Even if you don’t live in our corner of the world, even ideas about what sorts of suppliers to look for, would be helpful. Thanks!