any hardware store. lowes, home depot...much cheaper and with what potroast said about roots laying in the bottom, you also will get more volume of flow (wider flood in the square pipe), easier to cut your net cups holes in the tops and they will sit level no matter what. would be hard to get them to stand up real well in a circle pipe i would think. My misters are run through the top of the square tubing and spray down on the cups and hydroton. there fore the flood will never stop your spray and prevent clogs from other water and roots laying over your misters later on when they get big..... hope this helps...i would redesign just because there is already an easier, more efficient way.