Klockit.com and ClockParts.com each sell replacement clock batteries. Each retailer sells a variety of replacement batteries and other parts used in clock repair and construction.
Klockit is an online retailer that specializes in parts for clock building and restoration. Klockit sells the full range of standard alkaline and lithium batteries from Energizer, as well as silver oxide batteries for mini-clock inserts.
Clockparts.com is another retailer of clock hardware carrying the standard selection of alkaline and lithium batteries including N-Cell batteries, which are typically used for desk clocks and other small appliances.
Both retailers offer technical support, informational blogs and clock repair FAQs.