What I do most often is buy the 2-gallon food grade HPDE buckets from Home Depot or Lowe's and then I drill a hole just far enough from the bottom to install a bottling spigot. The bucket is like $5, the lid is $3, and the bottling spigot's are about $2 from Amazon = $10 per fermenter (plus tax). The excess CO
2 will bleed out through the not-quite-airtight seal around the lid. Or if you want to get fancy, you can drill a hole in the lid and then add a grommet and an airlock ... that might get you to $13 per fermenter.
My cousin owns an ice cream / candy parlor --- if I lived closer, I could get all the food-grade buckets I could ever need from him in all sorts of 1-, 2-, and 3-ish gallon sizes. So, I'll second BrewBama's recommendation to make friends will a local grocery store, bakery, or other small business.
And much like Kevin, I also sometimes save conveniently sized PET or HDPE jars and bottles from food products. The PET jars that peanut butter powder comes in, for example, are about a gallon. Publix used to sell organic apple juice in 1-gallon glass carboys, but I haven't seen those recently....
And I'm running an experiment letting two small batches sit in the 5-qt drink dispensers from Walmat. They're made of PVC; so the O
2 permeability should be quite low for a plastic.
And finally, I've also done femto-batches (that would the next thing smaller than pico-brew, right?) in large mason jars with the silicone airlocks designed for pickling.
Edit: These are $5 at Walmart:
https://www.walmart.com/ip/5065560927