I think the most common container for this need is the five gallon bucket. Bakeries and delis both get large numbers of these buckets and are usually happy to give them away. Drill a few holes in the bottom and you have a container that can handle nearly any vegetable.
Something else that might be available from delis are the round wooden boxes that large cheese wheels are shipped in. They wouldn't be a permanent solutions, but could carry you through a season.
A lot of the answer depends on your area. Is there any large industry in your area? If so, what items do they use up and how do they receive those items. Often things are shipped in 'disposable' containers that can be easily used as a planter. Even things like feed bags can hold soil for a season.
I have found planters at yard sales. I have all seen lots of items rescued from the trash and turned into planters. When the city I was living in changed recycling bins many of the old ones became planters all over the city. Wooden drawers, plastic tubs, 5 gallon buckets, old bath tubs and other items have been used as planters.
Any plastic container, for small things I use cutoff soda bottler and yogurt pots. Larger containers are old buckets (any decorator will happily give you hundreds of empty paint buckets), or old trugs (old cement mixing trugs from builders). Holed buckets are little use to the construction trade, but fine for planting in.
Really big containers are blue barrels (255L for £9), or ibcs (£20 delivered here).
Livestock minerals are often sold in big blue plastic tubs that are stackable and tend to accumulate on farms that have been around for awhile. They are usable for small scale aquaculture or planters (with holes drilled into the bottom). They will last for many years, even when sitting in full sun.