On a few occasions I had to get compost to get the new garden going, or to refresh it each year. Miramar landfill has a great compost program that's cheap. You can load it yourself for free, or for a bit more money, get the skiploader to dump a cubic yard into your truck. I've done that a few times. It makes my truck feel pretty wobbly but I know the surface roads to home. I just have to remember to expose the looped straps that extend the usefulness of the truck bed's corner eyehooks. Not doing so means it can be rather hard to get the tarp tied down!
A note on Miramar's compost: get the premium stuff that has been cooked out and turned the most. The lesser stuff is tempting but I got one load that turned my garden into a patch of grass that was a devil to weed all over again!