We bought an inexpensive arch from a traditional garden supply company online - I think we paid $179 + shipping. Lightweight, cedar (popular in our area for beach weddings). Though it's not the sturdiest, it assembls quickly & stands on flat surfaces very well - we've had it for a year and have rented it 4 times. Many are available in white wood as well.
Chuppah we made from PVC, fully covered with loose fabric - it's time to think about something new, though requests are few & far between. We made it in 2006 & have used it 3 times. It's difficult to set up indoors, but it works great in the sand.