It might sound daft, but I found scaffold boards a good source of wide planks. All graded, good stuff, so it shouldnt split, though not hardwood.
Plenty of meat for planing to thickness too. I think they were well over 1 inch I guess they are 9-10inch or so wide.
I made the steps for my living van out of ones bought new at B&Q. They were not cheap, but they were off the shelf, easy peasy to find.