I live in an apartment. In an apartment complex, you may or may not have access to the water heater water.

If you have advance warning, you can fill up your tub. But check now that your tub actually seals and will hold water. One of those large, round flat seals that simply lays over the drain is a way to be sure.

Put water bottles in the door of the freezer. 1/2" of air space is enough to keep them from bursting when they freeze. They'll help moderate the temperature changes of opening the freezer, and they'll help keep it cold longer when the power is out.

I have several refilled commercial bottles lining one side of a shelf of my fridge. When I use and refill one, it goes to the back and the others shift forward.

Between the freezer and the fridge, there's about 2.5G of water right there.

I have one of those 6 gallon plastic water jerries in the back of the closet, and plan to add another one. My "shoe shelf" is in front of it, and they all sit under my shirts very nicely.

I live alone, and that'll be a week's worth of water with very little storage impact at all.

Remember the space under your bed. A case of commercial bottles would probably slip under there, along with a LOT of stored food/supplies.

Remember the dead space behind towels/linens/etc. in the linen closet, and in the very backs of cabinets. Those are great for things you don't need access to very often. Put the stuff in front in low wire baskets so it's easy to move out of the way.
Okey-dokey. What's plan B?