Open the insert carefully (keep the spring and pivot from getting lost). Remove the old original batteries (mostly batteries and contacts are oxidized, clean them and the light flashes again).
When the batteries are empty or the bulb is damaged you need the following:
2x batteries VARTA UNIVERSAL ALKALINE No. 4061 - 1.5 V - LR 61.
They are available in photo shops or phamacies and cost about 4€.
Lamp: flashlight bulb 2.7 V, 0.3 A for 2 mignon costs about 2 €.
(Excerpt from a lexicon contribution to zipposammler.net)