I did an internet search on "who makes Costco batteries" and came up with Johnson Controls. They make Kirkland brand, they also make Interstate brand.
In my mind, going to Costco for your batteries is a smart smart move. Your battery box has two connections built in, and it should also come with one or two connectors that connect between 6V or 12V batteries.
If you tell them to take their battery charge off, you are buying at Costco, they will likely price match to Costco with no fee to install and you will have saved yourself a couple of hundred to enjoy.