With all the containers going to waste in the trash, I just can't bring myself to buy pots to plant into. I have friends that buy pots at Walmart, Home Depot, Target, etc. It goes against my grain, especially since self reliance/self sufficiency is my aim.
Depending upon the type of seed or plant, there are all sort of possibilities out there.
The paper ones can be used to start beets, spinach, chard, the sort of thing. Even cabbage, broccoli, etc. When the sprouted seed is just starting to make its first true leaves, I will gently break the carton into each little section. Gently plant each one. You don't disturb the roots this way. If I have been keeping the egg carton moist, never letting it dry out, the paper is soft and quickly rots away allowing the growing roots to escape.
The plastic ones are good too. I cut the top from the bottom and use both. Using a knife, I cut slits in sections so that excess water drains out. I start seeds in these that I intend to transplant to larger pots, for either selling or allowing them to grow bigger before planting out into the garden. Tomatoes, peppers, eggplant, flowers, herbs all do well.
Plastic bottles and containers:
I create drainage by making holes in the bottom. I have a small soldering tool that quickly burns small holes. A friend says that he uses a drill. That would work fine too. Plastic containers of all sizes and shapes can be used for pots. For ones that are odd shapes making it difficult to remove a growing plant, I simply make a slit down one side before putting the soil in. Holding the slit together with a rubber band or piece of blue painting tape, the slit can be opened at transplanting time. Simple. But nowadays I have so many old cottage cheese, and the like, conatiners that I no longer need to deal with the more difficult ones.
Plastic soda bottles:
These I cut the tops off of and put holes in the bottom. Some are shaped better than others for using as pots. The big two liter ones can make good pots, but I tend to use then to make fly catchers, fruit fly traps, and drip wateres.
Gallon Milk Jugs:
I convert a number of these into hydroponic units for growing leaf lettuce. I've given possibly a hundred away, trying to introduce the idea to others. But I also use them for pots. I cut the top off then burn holes in the bottom. I use these for plant starts that intend to sell. Right now I have maybe about 50 planted with onions. By the way, the half gallon and quart sized containers make good pots too.
Number Six Cans (well, I think that's what they are called):
You know the big jumbo can that bulk canned veggies come in? Well, that's what I'm taking about. The restaurants use these big cans, so I have a constant source. Using a bottle opener, I punch four holes in the bottom. While I can use them for all sorts of plants, I plant sweet potato cuttings and taro in them for resale.
Smaller sized cans cans can also be used as pots, but I find that they need a bit more attention in their use. I use the smaller sizes for plant starts I put in the shade. The sun heats up those cans rather quickly.
Used Coffee Bags:
My friends save me their used coffee bags, so I usually get the two pound size that Costco (Kirkland) coffee comes in. But I can use anything from 8 ounces and up. Using a scissors, I'll make a couple holes in the bottom. Now the bag is just like those black grow bags you buy! I use them for sweet potato cuttings, tomato cuttings, starting avocado seeds.
Any size bucket, or bucket like container, makes a good pot for larger plants.
Sturdy Plastic Bag:
The bags that large sized dry cat food comes in can be used for plants too. Not all brands use plastic, but some do. Same with dog food and livestock feed. Cut down to proper height, these I use for banana trees that I plan to sell.
I know that there are ways to make pots out of compressed peat moss. And another out of rolled newspaper. I've heard of people who will cut toilet paper tubes and paper towel tubes into sections to use as little pots. I've seen pots made out of compressed cow mature. I know of a person who reuses old pcv piping to make pots by cutting it into suitable lengths. I've never tried these methods simply because of the time factor plus the fact that I have a seemingly unlimited source of pot material already. But I'd enjoy hearing from other gardeners of how they get creative in making pots.
Do I reuse discarded commercial pots? Sure! I just clean them well and disinfect them first. I'll reuse them until they finally start to break into pieces.
What about terra cotta? I love those pots, but they are very hard to find in Hawaii. I will even use broken ones, breaking them up with a hammer and adding the pieces to the garden soil. I cherish the large ones, making them into ollas for watering in dry areas. But that's another story.