A good bow is the result of many hours of work. You can construct a suitable short-term bow fairly easily. When it loses its spring or breaks, you can replace it.
Select a green sapling or branch about 2 inches in diameter and 5 feet long that is free of knots or limbs. Carefully scrape the large end down until it has the same pull as the small end if you have the time to do so. You can leave it rounded for field expedient use.
Always scrape from the side that faces you, or the bow will break the first time you pull it. Dead, dry wood is preferable to green wood. To increase the pull, lash a second bow to the first, front to front, forming an "X" when viewed from the side. Attach the tips of the bows with cordage and only use a bowstring on one bow.
Select arrows from the straightest dry sticks available. The arrows should be about half as long as the bow. View my post on how to make arrows.
Make one or two more bows as a back up if needed.
Stay Prepared! Stay Alive!