It's the Advance Outdoor Heavy-Duty 10x20 Carport. We got ours from Walmart, but it's available at Amazon, too, for less than we paid:
They are available with sides for more $$. The posts should be fastened down; my son bolted his to concrete blocks. I used metal fenceposts and tied the carport poles to them with 2x4 blocks and U-bolts. Our cover was fraying after four years, so we bought a new one from Advance (which took some finding).
We originally bought the canopy for our Casita 17; it's a little short for the E21C, but not too much. The height extends to 11' at the center. Has survived snow and wind fine; keeps the leaves and tree junk off the trailer, which is a boon.