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.
For the 6.5 carboy I am using now (just started fermentation) how much krausen would be created? The reason I ask is that so far it doesn't look like it will go any higher than the 5 gallon mark.
During my 2 5 gallon extract batches (6.5 gallon carboy) the krausen has always gone up about 2-3 inches. Still lots of room above. That being said, I'd love to ditch the carboys.