Actually, it often is easier to build from scratch than convert from another band.
What do you plan to do with this antenna? Repeaters and simplex FM, or DX?
Most DX activity on 6 is either CW or SSB--and both are normally done with Horizontal polarization. A 3 or 4 element beam is usually a good choice-- on 6 they are fairly small and easy to rotate. They can be easy to build, too:
http://www.shelbrook.com/~ve7ca/AntA6m.htm
FM (repeaters and simplex) and the small amount of packet on 6 typically use vertical polarization.
If you don't mind instructions from a guy who drives on the wrong side of the road and mixes inch and metric measurements, this is a pretty easy antenna to build:
http://www.uksmg.org/vertical.htm
I used one of these antennas for several years on 6. It does very well on all the local repeaters, and about five years ago I made a 75 mile simplex QSO during Field Day using 10 watts( the guy on the other end had a 1/4 wave vertical).
I have since dismantled it and used the tubing for other projects.
The bottom loading coil can be wound on a piece of broomstick, and tuned for low SWR. Or, better yet, use an antenna analyzer if you can find one for 6 meters. The tubing is standard size and easy to find at hardware stores.
Dave_W6DPS