Broken windsurfing masts can provide good qulaity cheap tubes. To save time and weight I ordered the following from SuperSpars UK (http://www.superspars.co.uk). This cost around 500 GBP inc Tax and delivery to Austria.
Note: These dimensions are a guess and so some may be over spec and some may break!. My wing bar arrangement is NOT convertional. Wait till you see photos... Therefore these parts are only valid for a my arrangement.