It may sound a bit unsanitary but using disposable latex gloves I collected several hundred used communion cups from church and took them home. I then soaked them in pure bleach for several days and them boiled them in a pan of water.
Yogurts, puddings, applesauce, jellies also come in nice little plastic containers that come in handy for paints.
Also, most dollar stores sell a bag of 50 communion-like cups for $1. They are a little bit larger. Its not free but close.