Went through the very same thing.
Most auto part stores have the (Help, NeedA) section where there’s things like door pins, handles, knobs, ect. I found the petcock and the little threaded piece it screws into together in the same package in this section of a o'reilly auto parts. i know AutoZone has a section like this as well.
DORMAN Part # 61121
That number can be used at AutoZone or O'reilly auto parts, as well as anywhere that carries dorman products
when i replaced mine i found that the threaded piece that snaps into the hole was a little tight on my radiator i had to ever so slightly file out the corners in the hole in the radiator to make the new piece fit. if you don’t understand what i mean by the threaded piece you will understand when you get the new petcock.