Bird food does go stale, so unless you have a lot of birds, some would probably go to waste in getting a super large bag.
You could check on some of the sites that sell bird food to see what they have, may be your best bet
You could freeze it,but i wouldn't do it for longer than 6 moths to be safe.
I myself freeze harrison's pellets for 2/3 months.
Most online petwebshops carry large bags.
I buy most foods in 20-35 lbs at a time, but I also have a small flock of birds. With the exception of the bourke, they all eat the same pellets! (and speaking of bourke, I may switch her back to the other pellets soon - we'll see) The cockatiels get the bulk amount of seeds and I'm switching Jayde back to Nutriberries from Goldenfeast. I haven't committed to buying Nutriberries in bulk just yet!
Different brands may sell in 1, 2, 3 and 5 lb bags. Some have 10 lb bags, some have 15, 20, 25, 30 and 35 lb bags. After that it's usually 50 lbs and 100 lbs.
For a single bird, you really shouldn't need to buy more than 3-5 lbs at a time, max 10 lbs!
Some stores do sell from bulk bins. If you are going to a pet store and getting from a bulk bin, it's probably not a good idea... you don't know what bugs have had access to that food, let alone what people have touched, then put their hands into the bin. If you buy from an online store, that might be a different story because they may have better control of who and what is near their bulk food. One store that sells like this is MSBS.