Look here for info on the batteries.
D battery - Wikipedia, the free encyclopedia
You can always find/make a battery eliminator to replace the D cells. Than use the 12VDC adapter.
How to Make Battery Eliminators - wikiHow
Keep in mind that just because the unit uses 8 x 1.5volt cells that doesn't mean the DC plug on the side was not some other voltage. You might even be over powering it. It might be designed to use the 8 large cells for the capacity rather than the voltage. They can always step the voltage down inside the unit.
Can you give us the make and model of the unit? Maybe we can do some research and find one just like it some where? Are there any specs on the case anywhere? Be sure to look closely as many times the voltage specs can be small and simply molded into the plastic case.