Q&A for How to Build a Halfpipe or Ramp

Any home improvement store.

Of course, but 20x4 wood would be a bit hard to find at a store.

You can wet it (soak it in water because doing so makes the wood more flexible) and then apply glue on the edge of the wood that is going to be the side of the ramp. Then gradually nail or screw it on, making sure to clamp it down or press it in place with a weight so that it takes the form that you want it to have. Be careful, though, as you may break it if you don't proceed patiently or if you wet it too little.

Anywhere from $200 - $700, depending on whether it is outside or inside and what you will use to protect the wood.