Tritium (pron.: // or //; symbol T or 3H, also known as hydrogen-3) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (by far the most abundant hydrogen isotope) contains one proton and no neutrons. Naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays. The name of this isotope is formed from the Greek word "tritos" meaning "third".
Please read the Tritium Illumination article on Wikipedia to understand more about tritium vials. Check if possession of tritium is permitted in your country.
This is the short version -the linked Wikipedia article has more info.
Direct, short-term exposure to small amounts of tritium is relatively harmless. If a tritium tube should break, one should leave the area and allow the gas to diffuse into the air. Tritium exists naturally in the environment, but in very small quantities.
Due to the 3D printing process, some cleaning up of the hole/slot for the tritium vial will most probably be required.
I clean my models with a simple battery powered drill. It pays to work your way up to the final hole size.
Holes for a 3mm vial is 3.25mm on the CAD model, but the inner surface cannot be polished and hence it is quite rough. Furthermore the production tolerance for the vials means they are occasionally wider than 3.00mm.
I start with a 3.0mm drill bit and finish with a 3.2mm bit.
I pay close attention to aligning the drill bit with the hole -especially when cleaning multi-vial spaces (where several vials rest close to each other). The vials should slip in flawlessly. Do NOT use force of any kind. If a vial does not slide in easily; clean the hole again.
Other stuff you will/might need
I get my tritium vials from Steve Ku at Veleno Designs. Also check out his Facebook page -he does amazing stuff and (unlike me) is a craftsman and an artist.
Contact Steve at firstname.lastname@example.org.
The vials are best fixed with Norland Optical Adhesive 61 but I've also used super glue with good results (only on the ends of the vials, as the glue isn't all that transparent). There are a number of guides on how to use Norland 61 available on the web.