I haven't done much E6 lately, only a few dozen rolls, but have used Merrill-David in Cleveland, OH and Iris Photographics in Asheville, NC within the last year. Merrill-David was recommended by a local pro commercial photog that I respect, and he says there's no other decent local lab. He's used Merrill-David for many years.
Iris did a dozen rolls of mine this last spring and came out spotless and very well done. A long time photo buddy of mine has used them for nearly 20 years, and of countless 120 and 35mm rolls they've done for him in that time, he reports having perhaps 6 frames that were somehow spoiled.
Both will do E-6 by mail.
Merrill David Inc
3420 Prospect Ave E
Cleveland, OH 44115
110 Merrimon Avenue, Asheville, NC 28801
Hours: 9:00-6:00 Monday-Friday 10:00-4:00 Saturday