http://www.faunaeur.org/distribution_table.php

I hope I was helpful, but it's for Europe... :(

Try these databases:

http://www.marinespecies.org

http://www.gbif.org/

They have distribution maps for many species and also a list of references you might use to get in contact with some authors working with that species in the US and other countries aswell.

If you are willing to collect some individuals it would be very helpful to contact someone from the specific area who has experience with this species because, eventhough they might be present in some extense areas, Gammarus species tend to have certain habitat preferences and I am sure another colleague from the area can show you spots where you can find them in high numbers.

Good luck!

Lots in the Chesapeake Bay.

Thanks for the suggestions!