you might find a cattle farmer who would let you shovel cow patties from his field. if so you could get the old well aged stuff that way it wont be too hot. composted cow manure has excellent water retention and drainage.

another idea is to go out into the forest and rake up leaf litter. that combined with cow manure would be a great mix. be sure to use a little lime.

i dont know about your area but here there are lots of creeks that have great soil on their banks. you could go out to some place like that and shovel up some topsoil.

Hi daikon_tomato - it's a BIG container but it still is basically a container so you might find that a soil-less container mix works best - just as it would for regular small containers. You can always add compost to that of course but MG Garden Soil says right on the bag that it isn't for containers. As others have reported here last year, just like most dirt or dirt-based products, it tends to compact badly and doesn't drain well. It works best used as an in-ground soil amendment.

Over on the Container Gardening forum you'll find several recipes for good container mixes using peat, vermiculite or perlite, compost, sand, etc. You might also want to check out what is called Mel's Mix on the Square Foot gardening forum as that is the focus of their gardening - 4x8' raised bed containers just as you are planning.

Just some thought to consider. ;) Enjoy your gardening.

Dave