Certified GKH Gardening Expert
Whatever items you have on hand that will hold soil should suffice. This article should also help: https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm
Note: The pressure treated wood sold in today's stores are required to be non-toxic, so are therefore considered safe for gardening.
I used 2x6's stacked 1 on top of the other to make a 12 inch deep bed. Treated wood seems to work well for me. I love my raised garden bed.