OTG sets are White Poplar,. Southern Yellow Pine makes a great substitute, as it's roughly the same weight, has better resistance to impact damage, is cheap, and can be found in 4x4 at just about any lumber yard. I've played on Red Oak which is very durable but VERY heavy.
My favorite wood to use is Ash - it's quite a bit heavier than Poplar or Pine, but it's still lighter than Oak and is one of the most impact resistant woods there is (Baseball bats are Ash). I also happen to think its a beautiful wood, so I like to use oil rather than stain or paint to finish it.