You can make your own self-watering planters out of lots of different kinds of containers. Here’s a way to do it with a couple of plastic buckets.
The following is an excerpt from Fresh Food from Small Spaces: The Square-Inch Gardener’s Guide to Year-Round Growing, Fermenting, and Sprouting by R.J. Ruppenthal. It has been adapted for the Web.
You can make self-watering planters out of many types of containers. With the vast majority of large containers that you can adapt to planters, you will need to remember that they need to drain somehow. For example, a 5-gallon plastic bucket or metal pail may be just large enough to grow a single tomato or squash plant, or a handful of strawberry or spinach plants. These buckets do not come with holes near the bottom, so you will need to drill at least one hole for proper drainage. Many of the clay pots sold in garden centers do not have drainage holes either, so vegetables grown in these either die of root rot (with heavy watering and wet soil at the bottom) or they underproduce due to stress or dehydration (with cautious watering). It is a lot tougher to keep the soil moisture in balance when you use a container that has no self-watering chamber or wicking system.
To make a self-watering planter out of plastic buckets, start with two 5-gallon buckets. This is the standard size for paint buckets or bulk storage, though you should not reuse any bucket for food growing if it has earlier contained a chemical-based substance. Make sure that the two buckets are stackable so that one fits snugly inside the other. Ten-gallon, or any other size buckets, should work just as well.
When the buckets fit together, there will probably be a small space in the bottom between them, since the top bucket will not slide all the way into the bottom one. This space may be big enough for your water reservoir, or you may want one a little bigger, which can be accomplished by putting in a few small wood blocks or stones that raise the level of the top bucket another inch or two. You will then want to drill a drainage overflow hole in the outer bucket for the water, and you can locate this just above the top level of the water reservoir.
Since the inner bucket acts as the soil chamber, you will need only two more steps: (1) the soil foot/water wick, and (2) the watering tube. For the soil foot that will wick up the water, you can improvise again with whatever suitable item you can find; this could be a strong snack tray, pond basket, small plastic pot, or plastic food container. If you use a solid container, then you would need to drill some holes in each side to allow water to get into the soil that rests in this foot. Once you have a suitable soil foot, then place it on the bottom of the inner bucket and outline the shape with a marking pen. Then use a strong knife or saw to cut this shape out of the inner bucket’s bottom, place the foot holder into it, and fasten/glue/tape as needed to hold it in place (bear in mind that this part will be wet, so any glue or tape would need to hold up under wet conditions). Second, you should find a length of plastic pipe or tubing to use as the watering tube (again, a little taller than the bucket itself), and then cut another hole in the inner bucket’s base to accommodate this watering tube. The tube can be attached to the side of the bucket using a hot glue gun, stapler, or bolt and drill. As with the larger self-watering planter, the soil goes in the top chamber and the water in the bottom. As you add the soil, make sure to pack it tightly into the soil foot before filling the whole bucket with soil, as the tight soil foot will wick up the water most effectively. Plant your seeds or seedlings in the bucket and mulch the top as needed with leaves, straw, wood chips, or similar covering. Remember that the soil will dry out more quickly if not mulched, requiring more frequent watering. You could also cover the top with black plastic, in which case you would put down the plastic first, secure it to the rim with a rubber band, wire, or twine, and then cut spaces for your seeds or seedlings to fit. Another option to secure a plastic covering is to use the bucket’s own lid and cut out the entire inside with a knife, so that all you have left is a halo-shaped rim of the lid. Then fit this halo rim onto the bucket to secure the plastic covering. If you want a white bucket to retain more heat, you could paint it black or cover the sides with black plastic as well. (Be sure to let the plants breathe, at least through the drainage hole.)