ZF 1043 226 028 VALVE, ZF 1043 226 028 VENTIL looks like you can order it from Germany http://en.technikexpert.net/zf-1043-226-028-valve.html
5HP18 Valve Body – Valve Housing Groups, position item # 51.200, part number 1043 226 028, Valve 1 piece, used in Stuecklistennummer 1056 000 081/087/088