^ strong whining in many of the above posts..
Zygold's frozen bottle form egg whites are great. If you want a cup of egg whites a day, you buy 2. One goes in the freezer, leave the other one on your kitchen bench while you unpack your groceries and cook dinner, then chuck it in the fridge when you're done.
The next morning you have 250ml's defrosted. You then use it, when you have 1 day left, you move the other one to the freezer. I have never had an issue with them being frozen or bottled.
They only last 4 days, but that's 250ml a day eg 30g protein per serve...
ps. Anyway, to answer your questions: you would be most likely to buy them at a supermarket, or a supplement store and least likely at your gym (I believe everyone is dubious of products sold at gyms - they mainly sell marketing guff products like horley's).
I would sell it in a bottled form, not the flimsy carton.
Minimum 1L - above that would start getting dodgy with how long it could last in the fridge I imagine.