You can make du’a and ask Allah directly without making tawassul. It is more preferred that you ask by the right of Ahl ul Bayt (as) to grant your need. Also, you can make a Nazr (vow) that if your need is granted then you will do some good deed (specify the deed) and dedicate that reward to the 12th Imam (aj) for example or any other Infallibles (as).