Search for building supply stores.... Not home depot, but the kind that sell bricks, rocks, etc. They often sell both garden soil and compost by the cubic yard. And it is much cheaper. For example, my local nursery sells bags of compost (1.5 cubic feet) for $7. The local building supply sells pure compost by the cubic yard for $26. Thats like a 5x savings, you have to enquire as to what their compost is comprised of, but it is a much better bet than bags from home depot.

Another option is look into your county or city because a number have programs to sell or give away compost. Check with your local city recycling center or sanitation department. They often have that information.

Find a horse farm, a big one. They almost always have a gigantic pile of manure hidden in back of the farm with the back part of the pile fully composted. Usually, they can even load it for you if you have a truck, since most bigger farms have front end loaders.