Help me find this part...

[SiiiR]

Hi guys, today my car decided to not go in gear, could select certain gears but not reverse, 5th and 6th, gear stick feels alot lighter than normal, once i attach that part gear stick feels normal, but after a couple of gears popps off again, so clearly needs that white peice. i looked under the bonnet next to gear box and seen a part is missing, pictures below. Seems a white peice is missing by the looks of it, any ideas how and where i can get one. cheers