I have read about and started using plastic bottles (usually from water bottles). Yours are narrow so you might have to cut the bottle or get the half-sized plastic bottles, but I have been able to use the regular 16 fl oz bottles. For a few pots, I did have to crush and bend the bottle a little bit but for others, I was able to use it intact. I could generally put in about 2-3 bottles per pot, to give you an idea of the size of the pot.

Other people have also suggested that besides bottles, you can use soda cans for the same effects.

When you fill with soil, just shake it a bit and let the soil settle in between the bottles or cans. In this way, when you water, there will not be too much movement of soil to adjust to the actual uncovered spots in the pot.

I have used styrofoam peanuts in the bottom of large planters. They work great.

I like to use 3-4 gallon pots at the bottom of very large pots. Putting them upside down makes a mostly false bottom. They do not get as crushed as bottles or cans and not the mess to deal with at cleanup time as foam peanuts.

I've used pint-size grated cheese/takeout containers in some larger pots and it worked well. I just stacked them (with the lids on), like blocks, to the height that I wanted. When I emptied the pots at the end of the season, I just gathered up the containers and nested them for storage to reuse .