These are vendors that I use on a regular basis and highly recommend.
ReeferMadness
Dr. Mac and Sons
ReefScience
Big Al's Online
Premium Aquatics
I travel quite a bit for work, so I have the opportunity to see many local fish stores around the country, mainly midwest, east, and Canada. Here are a few that stuck out in my mind as having good marine sections. Some of these I haven't been to for years, so they may, or may not, still be good stores. Heck, they may not even be in business. This is just my opinion, based on personal observations. These are in no particular order.
Exotic Aquatics - near Baltimore, MD
The Barrier Reef - Boca Raton, FL
Aquarium Services Warehouse Outlets - Missisauga and Scarborough, Ontario
Pets Plus - Lockport, NY
Elmers Aquarium - Monroeville, PA
Wet Pets and Friends - McMurray, PA
The Reef Shoppe - Rochester, NY
Hoffers Tropic Life Pets - Milwaukee, WI
Sandy's Pet Shop - Louisville, KY
Uncle Bill's - Indianapolis, IN
Wet Pets, Eagan, MN
World of Fish, Minneapolis, MN
Monfort Aquarium - Cincinnati, OH
The Coral Connection - Willuoghby, OH
Aquatic Technology - Columbia Station, OH
Deep Sea, Inc - Kalamazoo, MI
Harbor Aquatics - Valparaiso, IN