there is no 2"x4"x9' ready oak in normal store. There will be lumber store that you have to level yourself with a planer.
The longest I saw is 1"x8"x10', or you need to find stud.
You have to know 9' is a very large tree and hard to make it straight. I suggest modify your design such that structure wise it won't need 9' stud. If you paint, you won't care about wood length either.
Many plywood is for underlayment, you can use them, but not good unless you cover with something smooth. You could cover plywood with 4x8 三夹板 though to solve the problem.
There are smaller sized plywood for shelves that you can paint. Not very large in size, 2x4 is the most common. This can be used for outside board as well.
