Where is the fun in buying an empty tin? A mint or two while you solder or fault trace and getting your next project box all at the same time! HiHi
Most boxes are not predrilled as position of the rf traces at decent frequencies are pretty critical, often mounting/soldering the sma connector +/- its coupling capacitor is the last act.
Sources for tin plate boxes might include Kuhne db6nt (EU) MiniKits (Aus) or Down East Microwave ('murica).