What ajiajs says. If you are looking for them for a good price, you likely won't find them any cheaper than from now until about Thanksgiving. Most likely, you will find them by visiting the stores however. They probably won't advertise them specifically. Some stores just mark down all their garden center stuff a certain percentage, but other just mark individual items down... Visiting the stores is the best way to get in on those kind of deals.

Walmart, Home Depot, Menards, Lowes, Target, Big Lots, Shopko, farm and fleet stores, etc....

Craigslist. Look for nursery pots and usually, there will be a lot of people selling large pots for cheap. But not surprisingly, you have to get out the boonies to buy them usually.

see if you can find large plastic barrels locally.. they're usually blue and can be cut in half to make two 25gal containers.. i can get them locally for around $20 each so turns out to be $10 for two containers after cut in half. if you want them to be pretty you can paint them.

I pick up large party buckets through this season time for cheap. On clearance between 2-5 bucks. I'm not sure how big they are, maybe 20-25 gal? But they are super sturdy, come with handles built in. And Halloween black and orange will be going on sale soon, Christmas red and green goes the first week of the new year. Some places get in Fourth of July too.

I was able to purchase big buckets for $6.95 at a local supermarket like the picture below. Home Depot sells them, too, but for another buck or so. I use this site to buy bulk containers. I have also found at the end of the season some of the local nursery stores will sell if not give away a bunch of them, too, same goes for landscapers.

I buy and use mainly the plastic poly bags. You can get half gallon to 30 gallons. I went to a hydro shop in another state on vacation and they said they did side by side comparisons of cheap bags to oxy pots and the bags won on every plant they planted. I just bought 10 20 gallon bags off Home Depot online. Shipped to store. Like 12 bucks. I am going to put my 5 gallon peppers overwintered into them in May.

I swear by them. Great drainage and peppers hate wet feet.

The local agricultural supply place here in central florida sell brand new 25s for about $4. 20s would probably go for $3.50. I don't know if you consider that cheap though. If so, I would at least check the closest one to you, just in case.