Google, or Remove it and look for a name inside. Take them to a local hardware store. I have had good luck with mom and pop hardware stores. They usually been in business 30-40-50 years and they either know where to get it or may have it collecting dust on a shelf somewhere.
try kings metal. Very good fast shipping and responds when I got my hinges from them....
They sells the locks too maybe check, one thing I notice when I refinish my gate was they are standard to the thickness of the gate you have. the pic looks like the Hookbolt Lock/Box Set they have but then they didn't have the bottom knob sets, so not sure... maybe you have some luck with search for it....
That lock was specifically made for that particular application by Adams Rite, last I knew it was no longer made. It couldn't hurt to contact Them & see what They say.