I have always purchased from https://www.treeprotectionsupply.com/
I have purchased Combitube Tubex, Photosynth, and the Tree Pro. All have their advantages. I think Combitubes will last the longest and provide the best protection when I spray. But for the value, the Tree Pro will be hard to beat.
I'd got with 5' for sure and 6' if you got extra money to spend.