Bigalsonline.com and thatpetplace.com have 2L jugs of Prime available, Big Al's has 4L jugs too. I can tell you that a 2L jug of Pond Prime is listed to treat twice as much water as a 2L jug of 'regular' Prime, how that translates back to an aquarium isn't necessarily that Pond is 2X the concentration of active ingredient.
How much water are you treating that you use a liter of Prime each month?! Granted I just ordered a 2L jug this morning, but that'll last me for... oh, uhm, years, which in retrospect has me rethinking that purchase a little, haha. I just couldn't stand to pay $10 for 250ml bottles when for $30 I could get 2L.