You can buy cables and connectors everywhere.
Can you tell us what it is for ?
If you have for example high power motor wires and also sensor wires, you better use two seperate cables.
Or if you want to have 12 sensors connected, you could use another Arduino to collect the data and transmit it over a smaller cable.
Please specify the length (10 meters or 1000 meters makes a difference), the current, the kind of signals.