I don't have enough soil for my pots, what can I use instead?

No.... the purpose for gardening in a large pot is to control the soil content and water ..... if you want the plants to do well use good quality potting or garden soil and fill the pot with the proper amount of it. ... the more junk you put in with the plant the less well it will do... and that wastes your money replacing the plants.

Plastic water bottles or soda bottles. Packing peanuts.

This is what I do: A layer of stones (for drainage and stability) a layer of Styrofoam (or card board or crumbles newspaper) and then soil and vermiculite, then the plants and sometimes a small layer of mulch on top.

do not use stones, that is a old way of filling pots and studies show the plants retain to much moisture....lay plastic water bottles half way, top with landscape fabric then add your potting mix

If you have a good potting soil, you could substitute top soil for some of it and also include some peat moss. If you use top soil, it will not have the nourishment that potting soil has or the amendments to hold more moisture and keep it from packing so hard. Make sure you fertilize.

If you had placed pop cans or a plastic garbage bag in the bottom you would not need so much soil.

Fill it with smaller pots first and add your soil on top.

Make your own compost from Kitchen scraps and tea/coffee grouts etc.