Is there anywhere nearby I can shop for nice evening gowns in Shanghai?

Yes! You can buy evening gowns at Huanlong Mall that located at No. 360, Meiyuan Road, Zhabei District. Walk north for about 400 yards to Xianxia Road Zunyi Road Station, and take bus line 127 to West Yan'an Road Kaixuan Road Station. Then, take metro line 3 or 4 from West Yan'an Road to Main Railway Station, Exit 6, and you can see it.
Or, you can take a taxi to the mall from the hotel directly. The fare may be CNY 30-40 for the single trip. Hope you like.