I root around our alley to find whatever nursurey pots that I can - in any size available for taking as long as they are reasonably sturdy. Pots used for small trees are the best - as they are generally thicker and have the heavy ring around the top making them easier to handle. When I find myself needing pots I generally reach for the "giant pots" in 5 gallon size. They are square and approximately 12" cubed. I like these because squares make a better use of the space and pack in to my greenhouse at a higher density, and they drain extremely well and evenly.
I've used the grow bag method before and find that you have to be very careful not to rip them. If you are in a situation where you can't justify the cost of pots, then go for the bags.
Scouting trash areas for pots or asking your neighbors or friends is the best way I have found for keeping costs down, and most of the time people are happy to be rid of them.