Getting 原廠電池 directly from Jebsen might be the safest bet but that also means an EXPENSIVE one. You'll probably pay a premium for the battery concerned.
Any reputable camera distributor (Man Shing, Wing Shing, Tin Cheung) should carry original Casio 原廠電池. Just provide the camera model and those staff should be able to locate one for you.
If you're on the HK side, try those camera stores along Stanley Street.