I've seen them at big discount stores like K-Mart or Wal-Mart. I don't know if they're still as popular as they were 10 or 20 years ago.

I got one from a pile of free items someone left in their yard after a yard sale. Maybe you can buy them for cheap at other yard sales.

You mean those plastic crates that look like old fashioned milk crates? Office Depot has crates something like that for file folders.

The stackable plastic crates...about 16 inches or so high? I bought mine at the Dollar General store...they have many uses...laundry, etcs. I have stacked them up, put an old door on top of them and used it for a potting bench.

I don't know what city you live in. but here in Detroit I go to my friendly neighborhood Gas stations and get the square (old milk crates" for free, they usally have a haphazard stack behind the gas station.

The Pooter!

The plastic "crates" sold in stores today are pretty flimsy compared to the ones used for milk. I have a real milk crate I got in 1987, it's very sturdy. But stores stopped putting them outside their stores, since people kept stealing them, and they are often returnable (to the milk supplier).

I use the plastic milk crates for stools when I am working in the yard (close to the ground, but not on the ground. Need to get some more, b/c I also use them in my greenhouse to stand plants on (off the ground) in the center, where i don't have shelves built (YET) - and also use them to run boards across to hold stuff i have potted up that does not need to go inside during the winter (gardenias, butterfly bushes, etc.)
I also use the plastic crates that drinks are delivered in (i.e., coke/pepsi products) to transport plants as needed - when going to a plant sale, plant swap, putting plants into the greenhouse for the winter, etc.
One of the former owners of the little store nearby let me have some of them, as well as the milk crates- they would leave them outside but the milk guy and the drink guy never picked them up....ask around....