Any Home Depot sells either planks or lumbers. You may want something like a 2" X 4"tacubaya said:
As far as I remember, wood ain't cheap. Visit a carpentry shop. You get 2nd quality Pine wood and ask them to cut it to pieces for you.
I'd use some diesel and a brush to coat them. The wood will absorb the diesel and will resist better to weather and bugs. This is a trick my old man taught me. They use this trick with light poles (my dad was a lineman). It's better with chapopote (tar) but that's hard to get and apply. Diesel will do just as fine.
Good luck!
Wish I could help designing and building.