I've seen people use concrete rings used in road drainage. I dont know where you'd get something like that though. Perhaps a 55 gallon plastic drum? You can get them on craigslist and local "bargain sheet" newspapers.

those mall ones are concrete and insanely heavy and expensive. Can you get a forklift to where you are going to put it?

your best bet is plastic or fabric. You can find large soft pots around the web. You can use a 55gal drum cut in half or you can use horse troughs, pond liners, etc. Then you just build a nice enclosure so it doesn't look terrible.

you may also want to consider your potting medium. A pot that large full of potting soil soaked with water is going to be very heavy. You may need to reinforce the floor. you might also want to consider a lightweight potting medium.

As already mentioned, the 55gal food grade drums cut in half make nice planters. I bought 1 for $10.

From your zone, I assume you may want to grow the ficus inside? If you're thinking of moving it outside for part of the year, your size would probably be limited to something under 30 gallons, like Ed's food grade drums or a whiskey barrel cut in half. Thats probably large enough for almost any ficus that would fit inside.

But if you want a really huge container that's inexpensive and doesn't need to be moved inside, you might consider smart pots. There are ones up to 400 gallons for $80. Not terribly attractive, though.

Here is a link that might be useful: Smart pots