Once established in tree pots, the plants can go up to any size including the ground.
Like Joe said, you can pick up pots for free if you know where and who to ask. I did that when I see people planting trees. And I also prefer the thick & strong injection molding pots. I also get buckets off FB as the big pots are more difficult to move. Durability of the buckets depends on the UV resistant materials on it if any. The rule of thumb is the ones that are originally designed for outdoor storage would have enough UV resistance to last a long time. The basic ones usually last 1-3 seasons, including the ones bought brand new at stores.
You can collect free buckets from donuts/cakes/sandwich shops or supermarkets. They are usually not cleaned so you need to spend time cleaning them.