Getting new tritium means paying $50,000 a gram, and you have to but the tritium every three months to keep the weapon operational.
IF you have a bomb that uses tritium -- a big "if", according to some reports I read a while back -- then the answer is, "No, you don't need to keep buying it every three months."
You only need to buy it once -- right before you plan on using it.
These things, if deployed as reported, are not a mini-strategic-nuclear-force, always maintained in a state of readiness. They are terror weapons.
Tell me -- prior to 911, did al qaida have to buy and maintain a fleet of jet airliners, doing regular engine and airframe maintenance, up until the moment of the attack?
A tactical nuke that needs a fresh dose of Tritium prior to detonation (and it can only be detonated once!) is an example of a "JIT-4-Terror" application.