Lets look at the 4 species and its significance. The Bible instructs us to collect these and wave them every day of the festival, except for Shabbat (Sabbath), so starting tomorrow (today is a High Sabbath until the first 3 stars come out tonight). As Christians we don't need to be kosher about all of this, so we shouldn't use up imported Jewish supplies and deny them the originals, since they are very legalistic about these things:

• The Etrog: Citron fruit. The ones with the stem still attached are considered the most valuable. They represent the heart and the fruit of the tree of knowledge in Eden. The name “Etrog” in Hebrew is an acronym for faith, repentance, healing and redemption. Also in Hebrew the initials of the words of (Psalm 36:12)
  Substitute: A large lemon, sweet orange, blood orange or mandarin (Etrog are mildly sweet and mandarins are also one of the original citrus fruits,) or an apple.
• Lulav: A ripe (green) date palm frond. Represents the backbone and they look for one that is study and straight. In Hebrew the word Lulav can be broken down into 2 parts “to him” and “heart”. A person who loves the Lord with all his heart, will be given “spiritual backbone,” real conviction and strength.
  Substitute: Any native palm.
• Myrtle branches: The Myrtle grows in tiers of 3 leaves each and you use 3 for the 4 species bundle. They are the shape of eyes and are associated with seeing and vision.
  Substitute: Branches from any tree with eye shaped leaves.
• Willow branches: In Hebrew called, Aravah – 2 leafy branches of a Willow tree (“willows of the brook”) they need a lot of water or they dry out. Often they are wrapped in a moist towel or the bundle is kept in water in the fridge when not in use, to last the week of Sukkot. Represent the service of the lips – prayer. On the last day of Sukkot they are beaten against the ground until many of the leaves fall out, this is intended to symbolize ultimate victory over your enemies. Since some people's mouths are their worst enemy I totally get that one.
  Substitute: Most places will have willow somewhere but any tree with long narrow leaves like lips.

The cluster of branches stays together with a special plaited holder made from the leaves of a lulav (palm). The 2 willow branches go on the left of the lulav, the 3 myrtle branches on the right. They are held in one hand while the fruit/etrog is held in the other hand. They are held together and waved before the Lord in a small ceremony every day of Sukkot, as a blessing is said, then waved in 6 directions, forward, right, to the back, left, up and down, to proclaim God covers us with protection in every direction.

Happy Sukkot – Rejoice in the Lord’s Providence and Presence.