"Grain of wheat" bulbs, fibre optic lighting, mini LED's etc, of the type used in model railways and doll's houses etc.
Look up miniature lighting and you should find something suitable.
Note that the rear lamp was located in the clear section of the redundant ventral gun hatch (the gun was not fitted), and not in the rear of the faired-off bomb bay opening, as stated by some sources. The front lamp was in the strike camera port in the bomb aimer's compartment.