I don't know, to be honest, how long you can store your batteries, because it depends when they were made and how long they sat in a warehouse. I believe not storing at full charge is best for the life of the battery. I would check their charge and store them at lower than full capacity (2/3 or so is what I have heard as best, but as stated above, opinions vary). You can check the charge every month or so if you are concerned. But I think three years is a fairly good estimate although honestly I'm not sure if there's one specified timeframe, only estimates.
I have also heard keeping them cool helps.
I have not had issues finding my preferred 18560 batteries. But I only use approved vendors in the US, they aren't showing up on "clearance" or anything but these batteries are used in many applications. It may be that some vendors feel the 27 series is the route of the future (and it may well be for many vapers) which may be why you are seeing that. I think that I am unlikely to switch to the newer battery sizes, I really have disliked that formfactor in my hand and weight. Maybe I am just used to what I am used to, but I'd like to see those mods around a while and doing better than what I'm currently seeing.
Best of luck,
Anna