Never knew they had farm specific batteries, but here is http://www.eastpenn-deka.com/default.aspx?pageid=490 and some of them in the picture are 6volt. I have used some of their 6 volt car batteries, seem to do well. Problem is they are in antique cars that get drive a few times during the summer, and never in the winter. Really hard on a battery not cycling it.
I think they have AGM 6volts too, however they are for golf carts and RV's. But if you are using it for a vibrator it might be an idea as they can handle the vibrations better, and less likely to spill acid on anytihng important. A lot longer reserve capacity too, so they would be better if she had friends over.