Look into the Hügelkultur: the mound method for home gardeners. It's basically mounding a bunch of yard waste, logs, mulch or whatever and covering it with soil and compost. Eventually it becomes compost and when you turn over your garden soil it's fertile. For Terra Preta, you can add biochar and terracotta shards to do as the Amazons did. If you are in a location where you can have a fire, you can make the biochar. Nurseries, Home Depot and Lowes will always have pottery shards if you ask. You might have to dig in a garbage can, but it's free.