These are ones I have used and would recommend, since I shoot 99% transparencies:

NYC: http://www.duggal.com/
Toronto: http://www.steichenlab.com/ and http://www.torontoimageworks.com/
LA: http://www.aandi.com/
Chicago: http://www.gammaphoto.com/
London, UK: http://www.metroimaging.co.uk/

And I found these two labs locally, but I haven't used them yet:
http://www.photolandonline.com/
http://milfordphoto.com/

Regards, Art.