> if tennis balls are hard to find in stores now, I will buy a case of 24 cans online

Standard panick reaction that's exacerbating the problem. People are piling up on all sorts of stuff right now. I needed new tyres for my mountain bike, largely impossible to find a few months ago. A neighbour told me "oh that's fine I can resell you a pair, I bought 6 a few weeks ago just in case" . Pair up tight supply chain and people virtually increasing the demand to stock up in case the offer runs dry, and you obtain magic.