Order a 7 gallon bucket from a homebrew site, local homebrew store, or, if you can find it get food grade plastic buckets from local restaurants. As long as you can get the smells out they are great and cheap.
a 5 gallon carboy will be messy if you try to ferment 5 gallons in it. it would be fine for 4 gallons though. Maybe brew up a big barley wine where you won't miss the extra gallon.