Problem with six speed cassettes is that theyre pretty rare these days. Unlike the standard that fits 7(with a spacer) to 10 speed cogs, the six speed freehub had threading on the outside of the body, the last cog serving as a lockring. If you ebay-it, you might find some really nice quality numbers in this format from the late 80s. Age matters little, but quality and spacing does. Makes sure its 135mm spaced (Im assuming thats what your rear spacing is) which is mtb and hybrid spacing.
Alternately get a rear hub with screw on threads and a 6 speed freewheel. If you shop around, this can be rediculously cheap. You can even ask on Usenet, some kind person might have what you need in their bucket of parts and give it to you. They do make 6 speed freewheels with modern ramps and pins, so the shifting will be great, and being thicker, those cogs will last long. Finally, when you sicken of gears, such a hub will take a track cog and you can go fixed gear.
Heres what a modern 6spd freewheel looks like, and goes for $20 over here:
