Best size wood for a project

[WoodenNickel]

I'm fixing the frame of a rotten screen door.  It would probably be easier to just buy a new one but I figure this will be a good learning experience.  The stiles are about 30mm x 54mm and I'll need to cut a rabbet on one side and a profile on the other.  A quick check at the Lowes website doesn't reveal any lumber that is close to the right dimensions.  So I'm trying to decide where to start.  If it matters, I'll be doing this with hand tools.  Should I just get a 2x4 and cut it down to size? Or is that crazy and should I ask a millwork shop to cut something for me and do the finishing touches myself?