I send all of my E6 slide film to The Black and White Lab (aka BWC Photo Imaging-South Lab).
In Dallas, phone 528-4200, otherwise phone toll-free +18004450264.
Just note that the prices quoted on the website for film developing are NOT correct. (for now)
E6 develop/sleeve/cut is still $9.05 for 36 exposures. Which means if you order mounted E6 slides, it still costs $11.00 (36 exp.).