I was able to find large plastic moving boxes -- 20 inches by 24 inches by 12 inches -- from U-Haul for $1.50 a piece. U-Haul doesn't throw in the dolly, but that's only $7 to $12 more to rent.

Large Plastic Boxes. Sterilite 64 Qt Latching Storage Box, Stackable Bin with Latch Lid, Plastic Container to Organize Clothes in Closet, Clear with White Lid, 12-Pack ...