There are few ways to get cages (half/full)
You do it the right way, welded on and complies with whatever regs (depends on what you're doing it for), CAMS, etc

Or the usual ones which are for show which doesn't do much to streghtening the chassis

If you're doing this, suggest you go and chat to the guys that actually build cages for race use as they will know the standards the car need to comply with.
CAMS would be a good start