Found a few more more.
There's a shop called Sure Smoke in North East Mall, in Hurst, near Nordstrom's. They are pricey though, 24.95 for a 10ml bottle with strengths up to 26 mg.
Dragon's Breath Smoke Shop in Euless normally carries juice too, Johnson Creek is what they mostly sell, from what they told me, but I'm not sure what the prices are. Apparently they're fairly popular, they've been selling out on a regular basis. Right now all they have is Clove in 18mg.
Also, any Smokie's House of Pipes sells liquid in both 18 mg and 24 mg strengths. Price is about $2.00 per ml. There's one on Camp Bowie in Fort Worth, one on Randoll Mill in Arlington, and one on Green Oaks in Arlington.
I know these aren't specifically in the Dallas area, but I figured the info could be useful.