We had to fit a new bed and replace cross members on our 1950 civilian Commer, luckily there is a timber yard about 15 miles away that has a large barn where all its offcuts are kept. The offcuts are all fairly big sizes and there are literally thousands of them of many different types of hardwood.
I managed to by enough offcut Ash to replace the whole bed of the lorry, mostly planks 150mm x 25mm by about 8ft. They do differ in size by up to 20mm in width and can also be shorter, some are 6ft. Anyway the planks were all individually priced and I paid no more that £20 for any single plank.
So, it does cost a bit but its well worth it. I plan on doing the same in the next few months to rebuild the body of our 1942 Commer Q2.