Buy a 6' staff, a saw and some glass paper. That's basically all the suppliers would do and they'd charge you a premium for it.
Head yourself over to Playwell.co.uk they have a selection of wooden weapons including staffs of varying lengths...
well if you havent already invested i would go to lowes hardware, they usually have small wooden staves for very very cheap..they do not have finish and are very plain but effective for use!
I was about to recommend Home Depot or some other DIY shop. That's where I used to buy my staffs, too. I'm only 5'6", and at least in Wah Lum we measure the size of a staff based on the individual. The way I was taught to measure a staff was to hold your arm straight overhead and curl your fingers. If you stand the staff upright next to you, you should just barely be able to curl your fingers over the top of the staff, cupping the top of the staff (and thus, being able to push down). So, a staff for my 6' tall son would be a bit longer than a staff that would be the appropriate size for me to use. I like the idea of the weapon being sized to the individual, but it's also nice just to be able to buy a "standard" sized weapon and use it.