RE: Battery sources in the Puget Sound area.
Usually Sams Club , Sears or one of the big retailers will have Golf Cart 6V batts at a good price.
I prefer Sears for a cruising boat as they are in so many towns .
Before any new house batts are installed its time for a SOC meter first!!!
You may find the existing ones are not shot , just mistreated , and living a month or two with a really weak set and SOC meter will teach you a huge amount of batt and load management , useful knowledge when not dock side.