You can pick up clover seed at almost any garden/farm supply store. It's usually sold as "Pasture booster" as a mix of red & white clover seed coated in little fertilizer beads. You'll need to aerate the soil a bit and water it down daily for a week or so, but once it takes clover establishes a deep taproot and will send runners everywhere. Make sure to let it flower and go to seed and you'll have more clover than you know what to do with.