There is no issue in this concern you may do this marriage. If anyone who believes in Oneness of Allah and believes that Prophet Muhammad (pbuh) his the last Prophet and believes in the hereafter is called Muslim and according to all Fuqha ( shia Jurists ) permanent marriage amongst Muslims are allowed doesn't matter which sect they are following.
Now if you want proof then our Aimmah's (a.s) life is the best example Imam Hasan (a.s) married w/o Jo'da binte Ash'as.Imam Mohammad Taqi al- Jawad (a.s) married with Ummul Fa daughter of Mamoon (Abbaside caliph).
there are many examples in history.
But you can't find any written proof in any book where has written that being a shia a person can't marry with a Sunni girl. Yes if are worry that you and your generation's faith would be affected your with this marriage then better not to do so otherwise there is no issue in doning such marriage.Later it would be your duty to convince your spouse to the faith of Ahlul bayt (a.s) to secure your future generations.