I know a lot of computer outlets sell them, but can’t seem to find an electronics store that sells usb cables that connect to the motherboard. An idea for a project I have would go inside the computer and be able to communicate over USB, and to do so it would be best to just hook up to the motherboard instead of looping around a cable to a plug. I imagine using header pins would suffice but it would probably be simpler to just have a plug that could go right in. Also, perhaps computer peripheral molex connectors instead of me buying extenders and cutting in half to solder?
find the name of the motherboard and the manufacturer. use that to find the user manual of the MB. Then look at the header definition. I think the layout of the header is fairly common, if not a standard. Get a female USB A connector and wire it up. You might get lucky and find an adaptor the MB manufacturer makes and sell but in my experience, they are hard to come by.
At the worst case, you can buzz out the connections. There are 4 wires, 5V, Gnd, D+, D-. You should be able to figure out +5 and Gnd. Try D+ and D- one way, if that doesn’t work, flip them around. Complicating it a bit is that the MB header usually has 2 USB ports on it.
Usually you just need a 2 row, 10 way female header/plug with a 0.1" pitch.
That will give you access to two USB ports, if you can find the pin-out for the mobo…
It’s almost always a standard in which case, use Google. It’s also a simple pin-out.