There are plenty of grocery stores which will give away those plastic buckets if you ask. They just throw them out.
A warning though, you are likely going to need to clean them yourself, but if all they’ve had in them was food, they’ll be fine for storing food.
I buy honey from local growers in 5 pound jars and save the jars and lids, which usually have a built in gasket. They’re great for storing food, too and free to boot, once the honey is gone.
The only plastic containers I throw out are like the 12 oz Pepsi bottles (when I even drink it).
But I have scads of the 1 qt gatorade bottles filled with water. And being a home brewer who likes to brew hard cider, I must have 50 plastic cider jugs full of drinking water.
When I use up a think of like laundry detergent, I just fill it with water and don’t even bother rinsing it. The reason?
Now I have some soapy water to clean stuff with!!
My rule is basically if it is in a jar or jug that originally held foodstuff, like OJ or cider or whatever, then its potable water.
Anything in other containers is utility water.