2 walmart 9v bats in series will be 5400mah
If the MV100 runs on 18v and draws 80ma then technically you should get (5400/80)*(.80)= 54hrs of run time from them
If you dont want to toss down the money or cant find the walmart batts tehn you should go for the rc batts they can be had a little cheaper and are easier to find the ones I use are 9.6v 2000mah they would run the MV100 for around 20hrs not as good as the walmart but not bad.
The problem is that its all dependent on the MV100 running on less than 24v externally, if its possible that it can your golden.