The Best US E-Liquid Made | DFWvapor.com, The Best US E-Liquid Made! looks to be about the biggest. They are located north of Dallas (actually two locations now, Lewisville and Frisco). I have not been there, but plan to visit soon. I have been to a few stores in the Mid cities area, the one I liked the most was Vapor Den (can be found on facebook), but it has just opened and not very well stocked yet. They have however just received some cool goodies, Vapaze products (AL-1, Nemises), El Capitan, and they carry Suicide Bunny ejuice (I'm patiently waiting for derailed to arrive as they are out atm).
the DFW area is HUGE and traffic is horrible (I hate it more than NYC) so be sure to schedule your trips accordingly. The Bush turnpike has helped a lot, but during rush hour it gets packed also. No toll booths, they will send a bill in the mail, not sure how that will work with a rental car... NTTA handles the passes if you want to go through the trouble (ACE cash express has them for a $40 minimum deposit). Avoid north part of LBJ (I635) if you can (that is why I stress the Bush turnpike) they are doing construction and the lanes are too narrow for all the crazy drivers here not to mention big trucks cannot fit between the lines adequately (I was a truck driver recently, 635 scared the hell out of me).
Good luck and hopefully someone that has done more local vape shop searching can give you some more info.