What sort of freewheel is it?

If it's a Suntour New Winner, you can buy a NOS freewheel body for $18 from Loose Screws (http://www.loosescrews.com/index.cgi...&id=3823127217). I haven't seen any other bodies for sale, probably because few of them are as modular as the last generation of Suntours.

If it's anything else, you cannot buy a body, and the cogs are almost certainly not compatible with any other model of freewheel. Shimano, for instance, use different, subtly incompatible spline patterns across their current range (to the best of my knowledge). Older screw-on freewheels will be even weirder, and documentation is almost impossible to find. If you are able to find a similar freewheel in good condition, though, you might be able to transplant some or all of the cogs to wind up with the correct set of ratios.

Most of the current freewheel offerings that still exist are:
IRD: 13-24, 13-28, 13-32 (about $50), 7sp, with similar 6sp options
Sunrace: 13-24, 13/14-28, many others (nothing close ratio, though) ($cheap, less than $20)
Shimano: 13/14-28 (I think), 13/14-34 MegaRange (also $cheap, but not quite so)

Rebuilding freewheels is such arcane deep magic that even Sheldon Brown says not to. It might be worth doing so if you have a really high quality close ratio freewheel (like a 13-18/19) and are willing to replace an unreasonable number of bearings individually in a procedure which sounds more like open heart surgery than bicycle maintenance. Instructions are on the same page where Sheldon says to never, ever resort to this.

That should be about all of your options. Happy deciding!