I wouldn't buy soil but rather compost. You've got soil already; what it lacks is organic matter. Clay can be quite decent once you have organic matter in it. Sometimes what you buy can be full of weed seeds but you can at least ask to see a sample before you purchase and ask what they put in their compost.

Or make your own, or lasagna garden, see the soil and composting forum here on Gardenweb for tips. I no longer bother digging compost in, I put down a layer of newspaper, pile up whatever organic stuff I can get. I'll work planting holes only, and in two years the bed is much improved. One year I was able to get the municipal leaf collectors to dump a truckload at the end of my driveway, and I grabbed all the disposed pumpkins I could to make it hot; threw coffee grounds from the local shop, egg shells, banana peels, grass clippings and rabbit poop in - nothing that would attract rats, raccoons or skunks. Spread it out about a foot deep after the winter, and planted right into it. Worked out quite well.