I've seen gaps in shops, but we get ours in bulk from Zooplus or Costco anyway, and they've had no issues, probably helped by Zooplus, as part of it's Brexit preparedness strategy, doubling its warehouse capacity, as it sells a lot of imported brands, so obviously foresaw the potential difficulties.
But we have 4 cats of our own and also foster, so are usually feeding around 8 cats, so it's worthwhile for us anyway.
Might not be a convenient option if you only have one or two, unless you're confident your cat won't take the arrival of several week's worth of it's favourite food as a cue to decide it won't eat it any more. But Zooplus do sell litter, so you could also get that delivered to make up the minimum order amount or even avoid the postage cost.
But I agree that it's harder with cats to ride out the 'well it doesn't exist in the shops so you'll have to make do with something different' problem because if you give them something that they decide they don't want today, they just won't eat it and make sure you know you've failed them.
It's one of those things that, unless you really don't have the money and/or space, this issue can be mitigated by not operating a just in time system yourself, because it exposes you to the weaknesses of the shops' just in time systems.
So instead of keeping little or nothing in stock, and buying what you need week to week, keep a small storecupboard of non perishable essentials (eg 2-4 weeks worth) and keep that topped up with your weekly shop. So if there's any time you can't get something, you just keep using up your stock, and hopefully you can buy double next time.
We've always shopped that way, mainly to take advantage of special offers and lower prices on bigger packs, and it doesn't take up much space or money, and is actually far cheaper in the long run.