My contractor friend knows of two in the area. A guy from church works for Martin Marietta Aggregates and gave me some names as well.You will not find too many mobile concrete batching tucks just because of the economics (low demand and high maintenance/acquisition costs).
For small loads they are pretty useful. The transit mix companies don't want to deal with small batches. It beats getting a pallet of bags from the home improvement store and mixing it in a mixer.