The branch connoisseurs that we are, we are often asked to identify branches. Here's a picture we recently received for analysis:
What's used here are Fresh Curly Willow Branches, which are typically available from late November through March. They can be bent fairly easily, making them a great choice for creating an arch.
If I were making this, I'd start with a bale of the 4-5' Fresh Curly Willow (10 bundles of 6-8 branches each). Then, I'd get a pair of attractive buckets or garden pots, fill them with plaster-of-paris or concrete, and set half a dozen or so branches in each one. Once the branches have set, I'd get out my cable ties, and start attaching branches until I have two columns that can be joined for the arch (each column would probably have to be at least 12 feet tall). Then I'd join the arch and put on the finishing touches, such as adding orchids (it looks like white dendrobiums were used here), and concealing the cable ties with something like ribbon or moss.