Sam's Club or Costco are usually good for 6V batteries. Sometimes local golf cart stores will have good or better prices as well, sometimes not--but it doesn't hurt to enquire.
Three house batteries is a rather atypical setup. Two 6V batteries may serve you better than three 12V batteries (particularly RV/Marine batteries), though it depends somewhat on your usage. There are also 12V golf cart batteries available (GC12 batteries); depending on the configuration of your battery compartment, they might fit and give greater capacity and better deep discharge characteristics. They're longer than 6V GC2 batteries, but similar in width and height, and a pair of them would have greater capacity than a pair of 6V batteries. They are usually less cost effective than 6V golf cart batteries for equal capacity.