Empty berry containers (new for me this year)
Lettuce containers (new for me this year)
Toilet paper rolls (I like these because they are tall, providing support for the plant)
Small nursery pots
Water bottles with the top cut off. You can tape it back on, creating a greenhouse.
Milk or juice jugs.
The possibilities are endless. Whichever one(s) you decide to use , just make sure you poke holes in the bottom for drainage. As for the toilet paper rolls, I would fold the bottom up a bit to hold the soil in.