might try and check witt the people at cardinal camping center in columbus ohio. they are putting in precast houses at the new ohio state grounds and they make them there. Mr. fishburn may be interested in selling some.
Because there is not much demand and the cost of transporting so high, they are pretty expensive for a one or two purchase. Plus, you need a crane to set them and your hole floor needs to be really level and well compacted.
The two we have built in the last year weren't to hard to form. after digging a 8.5 X 8.5 X 5 foot deep hole, we criscrossed rebar on one foot centers and bent them up about a foot. We then formed the floor using 2x4 with a little drop to the rear where we put in a 2x3 foot dry well under a drain hole to handle minor flooding. After the floor set, we attached a rebar grids for the wall wiring them to the up-turned rebar out of the floor edges.
Using 4x8 flake board we built a well crossbraced box that set so that the walls would overlap about 2 inches of the floor. The outside of the walls were formed by the dirt hole walls. We then formed the top of the walls using 2x6s. When we poured the concrete we set a row of 6x8x16 inch cinder blocks into the wet concrete on top of the walls and inserted an 18 inch piece of rebar in each cell. Later stacked 2 more rows of block. Should take about 4 yards of concrete.