Salvage yards and Scrap Metal yards are a good place to start!
Step 2: Chicken Wire
Cut chicken wire to double the size of the bike rim, with 2-4 inches extra on the edges. Using wire cutters, snip a quarter of the way up the length of the chicken wire to slip around the centre post of the rim. Slide the chicken wire through the spokes so there are spokes behind and spokes in front. Once you've slipped the chicken wire through the spokes, twist the snipped edges of the centre slit back together.
The chicken wire holds the garden to the bike rim, and helps keep the soil inside the rim.
Step 3: Landscape Fabric
Cut fabric to the size of the bike rim, plus a couple inches extra on all sides. Cut a slit halfway up the fabric in the middle to slip around the centre post of the rim. Slide the fabric through the spokes to sit directly on the chicken wire.
The fabric holds the soil in place.
Step 4: Soil
Add soil to the fabric, making sure the fabric's edges remain over the rim's edges so the soil doesn't fall out.
Step 5: Add Plants
Plant your chosen shade plants in the soil!
Step 6: Add Moss
Add moss to your garden to cover up the uncovered soil--this prevents the soil from falling out.
Step 7: Secure the Back
I built this garden on the tailgate of my truck. You can do it on a table, or anything with an edge that you can slide the rim partially off to get underneath to secure the back. Cut some of the extra chicken wire to use as ties to secure the chicken wire inside the garden to the back spokes of the garden.