Blender has a mirror function, it's in the modifiers panel. You have to be in object mode, it won't work in edit mode.If your unfamiliar with Blender, it's a free modeling program with many high end features and a good selection of export/import options. It's also notoriously user unfriendly with it's interface relying heavily on hotkeys.
If you have problems figuring it out, just let me know and I'll send you button by button instructions on how to perform the operation ;)
Cheers,
Chris