Where to go for a new Mainstreet office space?  That's the question for Port Huron city officials as they ponder going forward with a revitalized Mainstreet that would be focused on promotions and the other pillars of traditional support.  Blue Water Area Chamber of Commerce president Vickie Ledsworth says the chamber's welcome center already has the space that would be needed, without spending thousands of dollars on renovating another location.Ledsworth says in a recent meeting the need for a downtown welcome center with public restrooms, information center, a conference space and an office would be needed for a part-time director, and while local developer David Witt has offered the former Whipple Tree storefront, it would cost too much to rehabilitate.  Instead, she says the chamber is offering assistance by letting a new Mainstreet organization share space in the already existing downtown welcome center.