Some people use the plastic water jugs, but others (myself included) do not use them because of concerns of how they will react to the acidity and alcohol content. Another problem for me is that they are 5 US gallons (19 litres), and I usually make kits thus requiring 6 US gallons (23 litres).
they are the 3 gallon, and plastic. I cant find carboys in my area for under 30 bucks. everyone wants close to 40.
I would not use them myself. Some may be water grade but not graded for higher acidity like in wine and will leach chemicals when exposed to these acids. If you must I would only use them for very short term storage like initial fermentation. Get some glass, its worth it believe me. How are you going to feel when after 6 months of babying your pride and joy have it ruined by being cheap and not buying a glass carboy?