We can source it but we do not stock it by default. This is because Harrison's recommends that their bags are used up within six weeks of opening. Unless you have many birds, it'd be hard to get through such a bag. Our African Grey goes through a 455 g bag of High Potency Coarse in about three weeks. So a 2.27 kg bag may be more appropriate for two large birds. For small birds, we always recommend 455 g bags.
The Harrison’s bag is a very effective, re-sealable oxygen/moisture/light barrier. Its layering consists of metallic foil (keeps light and oxygen out) and plastic lining (keeps moisture out). Blocking all of these processes is critical to keeping a food fresh. The zipper lock allows you to squeeze out any oxygen inside the bag. Oxygen ions can pass right through plastic, rendering Tupperware and zippie bags less effective than the Harrison’s foil-lined bag.
Vitamins in general degrade quite easily in the presence of air, moisture, and light. It's fine to store for quite some time while it's still sealed, but once you've opened the bag, and as you open and close the bag every day, the food quality goes down. After a couple of months, you may notice that 'rancid' smell you get with old nuts, cereals, or grains. If no bugs have gotten into the food by then, it may be still ok to feed to your birds, but it certainly won't be the premium experience you paid for. Do the best by your bird, and use up the bag within six weeks, as recommended.