My local theaters seem pretty good about mixing the 2D with the 3D. Of course, neither RPX or LieMAX are local, but I do have other options for big: Cinerama and the real IMAX at the Pacific Science Center. The former of which will play 2D versions, but only on Tuesdays.
I kinda hope the local Regal gets an upgrade to RPX and Atmos at some point, but it's a downtown building so there may not be much possibility for renovation.