Tritium EXIT signs can be bought from various shops and on the internet, for example at:
These EXIT signs are self-luminescent and do not need any connection to an electrical source. They require no batteries (no maintenance) and they remain self-luminescent during 10 to 20 years.
Buying these signs is only possible in the US now (and a few other non-european countries). The signs don't comply with the new technical requirements in Canada (they are not bright enough to meet Canadian requirements), but it seems some canadian web sites still have them for sale now in 2007.
As a consequence of radiophobia, because they contain some radioactive tritium, these EXIT signs are forbidden in Europe now, although they are not really dangerous (as explained below).
In older times (half a century ago), alarm clock needles were painted with radium rich paint that glowed in the dark (green color). These are now forbidden as well, collected by government nuclear waste agencies and considered as hazardous nuclear waste. This is also a consequence of radiophobia to a large extent : few people eat their alarm clocks ! The only danger in this regard was for the workers doing the painting. Some of them in the Paris area in the mid-20th century used to lick their paintbrush several times per hour to make it more pointed and paint more precisely.
Now as concerns the EXIT signs :
The half life of tritium is 12.3 years. That's why the EXIT signs are given as OK for about 10 to 20 years of lighting (over a longer time span, the glow seriously diminishes, of course progressively).
The radiation level received from tritium EXIT signs is NIL as long as you don't break the EXIT sign because tritium is a BETA emitter, and BETA rays are stopped by the internal pieces of the device (the radiation doesn't get through the inside tubes).
Each sign contains about 15 tubes each of which has a small amount of tritium gas (non toxic amount even if it were entirely absorbed which would be a very difficult thing to do).
In case the sign is badly damaged, one (or more rarely, several) of the tubes might be broken. Radioactive tritium gas might then escape into the local area, but it can be dispersed easily just by simple ventilation.
One EXIT sign contains a total 25 curies (about 2.5 milligrams of tritium) in these 15 tubes.
While not impossible (one would have to break and ingest entirely the tritium gas from all 15 of the tubes contained in an EXIT sign), a large enough dose to cause significant harm or be lethal to a human person is very, very unlikely.
Tritium is a gas. When inhaled, it does not enter the bloodstream easily like oxygen does. It is almost entirely exhaled when the subject breathes out, just like nitrogen, only a very small fraction might be absorbed in this way. Although the lethal dose for tritium is about the amount contained in one EXIT sign, there is no simple way to absorb that tritium in the body.
There has never been a single case reported in the world where this has happened. It occasionnally happens of course that perhaps one or a few tubes (out of 15) might break and their content be partially inhaled (and most of it immediately exhaled at next breath), resulting in very minor doses absorbed. This is not toxic.
An easier way to get the tritium in the body would be to drink tritiated water where T replaces H (HTO or T2O), but the tritium in EXIT signs is not in this liquid form. It is in gaseous form (HT or T2, like H2). This makes it very difficult for the T to enter the body and remain there.
Tritium when absorbed by the human body is progressively eliminated by peeing (T replaces hydrogen in water).
The biological half life of tritium is about 10 days (because you would pee between 1 and 2 liters per day and the volume of circulating liquids in the body in first approximation is 6 liters blood + 12 liters lymph.
To decontaminate faster, if this exceptionnally rare event occurred : drinking more water makes you pee more, and eliminates H20, HTO and T2O more quickly (in proportion of the amount of water taken in and eliminated: drink 10 liters a day makes you eliminate 10 times faster than 1 liter a day).
So if you come across one of these signs, be careful not to break the sign, but even in this case, the risk is almost NIL (just ventilate the room, handle the broken sign with gloves and place the sign outdoors in the wind for a good while). Some radiation might escape in this case only, but poses no great danger, just be careful to ventilate and avoid breathing that air too much to the extent possible. There is no high risk however even in this low-probability case.