One advantage of fluorescent bulbs is that they last very long and therefore are cost-efficient. This particular model has a so-called G5 socket, so make doubly sure that it will fit.
From Amazon
Osram 591667 24 Watt Lumilux T5 High Output Fluorescent Tube Lamps
LED Bulbs
Osram T5 Fluorescent Tube 24W 550mm 21 Inch Warm White - 591667
Big on Electricals
Osram T5 Fluorescent Tube 24W 550mm 21 Inch Warm White - 591667