Yes my pet store have a lot. I have gotten some free from them just for asking. They are large and thick make good study pots.
My main purpose when I started this was to keep styro out of our landfill. But if you want to buy a box. They sell them at Dollar Generals stores, I see them at Family Dollar, and those One Dollar store.
Look around the summer paper plates and plastic fork area. they sell them for people to take food on picnics, and carry ice.
Although it is cheaper and more environment efficient if you can get your box free since you are in the dollar store you can pick up some decorative rocks and stones. If wood is your choice they have some nice wood pieces you could cover your box with wood and stain.
Either way buying the stuff to make the pots are still cheaper than if you purchased a pot from Lowes.