Overlanding's such a broad category.. mixing two or three very very well established institutions. (Depending on your individual aspirations for the hobby)
So, as far as off road stuff, there's hundreds of shops that will outfit your truck, soup to nuts, with body, suspension, electrical, and wheel and tire mods. This includes some "overlandy" stuff like rooftop tents, cabinetry, awnings, showers... Etc etc.
As far as retail stores for the gear carried inside... Once again there are hundreds of places that do camp shelter and kitchen supplies. Sporting goods stores and camping stores, that sort of thing.
If you're looking for a one-stop-shop that will crank your truck 4 inches higher and sell you a skottle.. I'd be interested to see one.
As much as it pains me to say, brick and mortar retail is a slowly dying industry, but you may be on to something...