generally, trade missions as well as mining missions are generated from demand. ideally you would want to have a station with not much else in any demand, but those don't exist afaik.
- so, factor a: you need a station with demand of tritium.
i personally have not seen a tritium MINING mission, only supply (fetch) missions. but if there are any, if the same logic applies as to all other mining missions, those have to be no boom-mining missions, as boom mining missions are only for only-mineable commodities, while none-boom-mining missions can be for commodities which you can as well buy.
- so, factor b: faction shouldn't be in boom.
if you don't mean wing-mining missions, because those can be for non-mining commodities even in boom.
- for a lack of other missions, my go-to place for non-boom mining or wing-mining missions are terraforming or sometimes industrial stations (those that don't have a secondary economy of refinery or extraction). some commanders swear on agriculture.
while i'm interested to know, whether tritium mining missions exist, i'm not sure why you need them. i'd prefer to deliver 728t of bought indite for measely 40 mio of an indite wing mining mission above 756t of trtium for mission max cap of 50 mio, which will cost you much more to purchase...