Amazon has Purely Products PUVHX213 13-Watt UV-C Germicidal Ultraviolet Replacement Bulb for $20.75 + super saver eligible.
Technical Details
Non-chemical approach to disinfection; Fits the GX23 base and has two molded snaps
At least 16,000 microwatt seconds per square centimeter but many units actually have a much higher dosage
Commonly found in laboratories, hospitals and clinics to sterilize surgical equipment
8,000 hours or more of normal usage
Prevents DNA of germs to reproduce