Uses of tritium in the US are regulated by the NRC. It's only partly a question of "how much tritium can you have in a civilian device". The actual applications are regulated. Tritium is permitted in watch markers and gun sights, but not permitted in "novelty" devices like glow rings. It's permitted in exit markers provided that the markers are permanently mounted at their location and that the location is registered with the NRC. I don't know if these map reading things would have any chance of getting approved, and it would probably take a mountain of paperwork to find out.
As an alternative, I suggest a Tektite Trek Lithium flashlight. That's an LED dive light (1AA sized) powered by a 3.6 volt computer battery with 10+ years of shelf life (tritium has about 12 years half life) and over 200 hours of runtime. It's completely waterproof and pretty much indestructable, and should put out much more light than the tritium thing.
A cheaper alternative is take a white-LED Photon II or similar coin cell light, and replace the two CR2016 cells with a single CR2032. That will make the light a lot dimmer, but it will still be adequate for map reading, and it will run for several days straight.