I don't think you are going to have much luck trying to find 72" reflectors and/or bulbs simply because of the limited demand for them. You don't see them a lot (if at all). What you could do for the reflectors is to use 2 36" clip on reflectors to go on a 72" bulb but not sure where you would even start hunting a 72" T5 bulb.
http://www.aquabuys.com/miva/merchan...Store_Code=aqb
Says that the 60" bulb is an 80W, so I am not sure what a 72" bulb would be.
Are you trying to do a DIY retro style kit on a 6 foot tank? If so, its easier to find the parts to do a 72" VHO setup.