Um.... why do you need a splitter?
Assuming it's going on the board in your sig then there's already a socket for it on the motherboard....bottom right just under fan, it's got usb3 underneath it.
You can get internal hubs for the header from the likes of Sabrent (not that we can really suggest where...)
SABRENT Internal USB 3.0 Hub/Splitter (HB-INTS)
There's some 'no name' cable options but honestly no idea on how they work
GINTOOYUN USB 3.0 Header Extension Cable
No idea if they'll have issues with speed/compatibility etc