If you have space, and in the city that’s not a given, rainwater collection barrels aren’t super expensive and you don’t have to use them the way they are intended. Put it in your basement or out the back door, fill it from the tap with a hose, and never connect it to anything. It’ll sit for 1-2 years and be just fine. To be clear I’m not suggesting you use it to collect rainwater. Just fill it from the tap for such a time as the tap stops working or what it produces becomes unsafe.
Second best are those stackable water containers. More work and more money for the same volume of storage, but more convenient to store.
How you’re going to practically use your water supply in an emergency is worth putting some thought into. When the mains water is unavailable, it is hard to explain just how annoying it becomes if you don’t have some arrangement of tap over drain to use. This is where the portable water totes with the taps come into their own - drag one over to the kitchen counter and position the tap over the sink, and it works great as a standin replacement for most tasks that need running water. Here’s the rainwater barrel’s greatest weakness - you’re not going to get any flow from it sitting on the ground or in the basement, so you need to plan to decant it to at least one portable container to make use of its contents.
Realistic budget is maybe 3 gal per person per day btw. It’s easy to underestimate this because we don’t really appreciate just how much water we use each day for all kinds of things.
We had an interruption to the house water supply last year that lasted a week, so this stuff is pretty fresh in my mind.