I would just use small plastic cups for cheap and then upgrade directly to their final home say 5 gallon containers so you only need to pot up once. I found buying small containers every 2 months and having to pot up 2-3 times a waste of time and money and puts un needed stress on the plant.
Well, I was going to recomend you check with a local nursery but everyone beat me to it. I picked up 20 3 gallon pots for 25c a piece. The owner was extremely suspicous and asked why I wanted them. I think he thought I was going to try to be his competition.