I've made a number of portable depthfinders for various boats. Just about any unit can fit the bill. I used a Plano tool box and mounted a board on which I screwed the bracket. The transducer I mounted to a piece of 1x3 or 1x4 I c-clamped to the transom. Then I soldered alligator clips to the power leads. Any battery could be used. The beauty of this was, with the proper design, I could put all the parts in the toolbox, close it up, and pack it away until next time.

I don't know if Cabela's would sell the packs separately, but I do know that their small sealed batteries are on sale right now with charger. Go to the fishing/marine section of the mid-winter sale on their website.