There are numerous researchers on both sides of the Atlantic who could do
research for you, but before you waste any money, you need to know the ancestral
cities and towns. The Hannover you have is probably the Prussian province,
not the city. Late 19th and 20th century ship arrival records should record the
towns. Also check marriage licenses/certificates and particularly any
Lutheran church records over here.
Once you know the towns, the rest is fairly easy and fun to do yourself.
John V. Heyse