I use to go to Patrick Bros near Wisbech. They own a demo company and sell salvage wood of various species. I paid 70 quid for a 8 inch x 8" x 8 ft long piece of old oak from a post and beam demo. They ripped it to 30mm thick on their band saw and I finished the wood. Awesome tight grain as its old , they had some in from France the last time I was there and a butt load of Pirana Pine 1 1/4" X roughly 12" various lengths. I used to have them cut me 8 ft lengths. They had a stack of what looked like 20 mm Luan Ply they were selling for 16 quid. They had pine, beech etc.
Like I said itdepends on what you want