I picked up two at my local NAPA. They have them with varying numbers of circuits. One I use for 6v accessories and one for 12v (from my 6-12v inverter). I mounted mine on the inside of the firewall, up high so you don’t kick it.
I bought one from Defender Marine. It has fuses that are different for the different circuits. I mounted it to the firewall, engine side, with a magnet glued to the backside so that I would not have to drill any holes. A piece of cloth protects the paint from the sharp edges of the magnet. I bought one with the cover. Each circuit is labeled.