Hello! It´s very hard for me to find a squared PVC

First, I would like to say that I really think your system is great and I´m very grateful for you to have posted it for all of us to see. I plan to build one of these vertical systems but I have a problem finding a fence post (or squared PVC) here in Mexico. So I was wondering if any of you have any other alternative to the PVC tube. Maybe a different material that is still healthy for the plants or anything like that. Please let me know if you have any other idea, I would really appreciate it!