I used RevolutionPly Hardwood Plywood and had it cut into 8" strips at Lowes Hardware. This is commonly used in cabinet making but works great for a cheap alternative shiplap. It comes in 8' x 4' sheets and is 1/4" thick and is only $13.47 per sheet. I was able to get 6 boards per sheet bringing the total per 8" board to roughly $2.25. Having them cut at the store is so much easier because they have a great set up for ripping and cutting boards.