For mine I picked up a slab of beech locally which had been stored for a couple of years so nice and dry. Just the right size for 3 shelves. The legs are oak from a local timber merchant, £10 for sawn offcuts. Oiled with Danish oil. Spiked shelves.
My son’s rack is sepele shelves and oak legs, again using offcuts from the timber merchant. Danish oil. Fixed shelves.