Straw is a common raw resource used to craft items and construct buildings and wonders. The easiest way to get straw is to farm it. It also can be found in barrels, crates and at the pearl shop.
Farmer's Tip: If you plant straw (or any short timed seed) in all of your farms and you have a farmer(s), leave the island immediately after sewing the last seed (great time to visit friends or other islands). When you return All your plants will be ready, with none wilted.