There are several ways to get timber, depending on how broke a goddamn writer you are: You can find it, forage it, steal it, or get something vintage from Etsy, which most of you will want to do because you're all goddamn hipsters.
I don't have time for that shit, so I bought mine from a shop.
I bought redwood, because that's what they had. I learned that the name redwood is a lie. I suppose it has a reddish quality if you squint, or have blood in your eyes, but it's mostly beige/yellow.
Because I have a tiny flat and am overly judgmental, I need a fairly shallow desk. I opted for two lengths of redwood that I could fix together to make a desktop 450mm wide. That's 45cm, or just wide enough to catch my falling tears as I fail at life.