Montauk Sofa, a Montreal-based maker of sustainable furniture since 1995, with clients including Brooke Shields and Calvin Klein, has bought a 4,500-square-foot retail condominium. It is on the ground level of a newly developed mixed-use luxury condo in a 1915 five-story building in the TriBeCa East Historic District. For the past two decades, the business, which has stores throughout North America, was at 51 Mercer Street in SoHo. Over the last few months it temporarily leased space at 39 Lispenard.