i took my tag to my regular jeweler for a cleaning and after 7 years, i only had to replace the battery once, so he did that as well. as long as they know because it's a dive watch it needs to be pressurized, you can probably take it to any jeweler.
there are many brick & mortar jewelry stores (i.e. bb&b) that sell tag that could easily service it for you. or just google tag in your area and see what comes up.