A:
THE BAD
Nobody sells one half of the mount...
THE BETTER
Can you contact the last person who left it and ask for the other half?
THE BEST
The existing mount might be securely anchored into the wall studs,
if so you can simply remove the old mount and use the existing bolts to mount the new one.
All wall mounts will mount to the existing holes as long as the weight rating is not excedded.
If mounted directly onto the wall studs, the weight rating will be much higher...