I use the rectangular-shaped water jugs we occasionally get from BJ's Wholesale Club as carboys. $5 for the bottle deposit. They are 5.28 gallons, and because of the rectangular shape they have a smaller footprint and just fit in a compact 'fridge I have set up for temperature-controlled fermentation. Because there isn't a huge amount of head space I use a blow-off tube.
On the bottom of these bottles is a "7" in the recycling logo. They also have the name Greif molded into the plastic on the bottom. I ran that down to a plastic bottle mfr., found their web site, and sent an email to their Sales & Marketing Manager who responded that the plastic is a polycarbonate resin, FDA approved for water, and he supposed they could be used for fermentation as well.