Here in Jersey it was Steve Vargas on Easton Ave in New Brunswick, Meyers in East Brunswick, then Edison. Those were my go to places early on. I do miss Sean Marchev/Dougherty's and Millburn Hobby too.
When I was out in Dayton, Oh, I remember a place i think it was on the Oakwood/Kettering border called Far Hills Station, as O gauge was beginning to intrigue me and this place had a good amount.
Some good shops still around here in my area, Big little Railroad Shop in Somerville, Model Railroad Shop in Piscataway and a bit of a stretch from home but close to work Train Station in Mountain Lakes