I believe you can, there has to be a distributor for them, since you can mail order stuff with tritium vials already installed, various weaponsights, and keyrings are two examples. If you can mail order stuff like that, shouldn't be a reason why you can't buy straight from the manufacturer. Asking a rep from Trijicon how they do it may be a good starting point.

I know you can't export it though, not without a special license. For example, if a soldier who is already deployed wanted to purchase an ACOG and have it shipped to him, they wouldn't be able to.

A little info on tritium. I remember someone mentioning once not liking the idea of having a radioactive isotope near thier important parts (ala a keychain). Turns out the radioactive particales cannot even penetrate the skin.

http://en.wikipedia.org/wiki/Tritium