DIY bus bar in box:

Three bus bars in three boxes (two (+) phase-one (-) phase)

I cut, drilled, and tapped a piece of copper bar I had. The bar was attached to HDPE standoffs, glued into the 4" weather-proof boxes.
You can buy something similar off the shelf. Just make sure your termination lugs on the bar are big enough to handle the bigger cables used for batteries. With a somewhat sophisticated multiple battery setup, it's easier to buy a hydraulic crimper, some bulk cable, and lugs to attach cables. Otherwise everything can be ordered off Genuinedeals.com but you may be waiting for a cable to complete your installation.