There are many places that will offer scanning of your negatives, the primary criteria that will determine where you should go are the quality you want and how much you want to spend. A high quality drum-scan of a 4x5 neg can cost you more than $40 per image! Professional labs will charge by the size of the file you want back (basically the resolution of the scan). If you want more than an 8-bit file, expect to pay a lot.
The two labs I use in Los Angeles are Icon (www.iconla.com) and A&I (www.aandi.com). Neither has "wowed" me with their prices, so I would consider looking around some more. As someone else pointed out, Ken Rockwell swears by NCPS, so they might be worth a shot. Also there are surely some good labs in NY, though I'm not too sure what they are, but I would try to find out who people are going to out there. For cheaper scans I'm sure there must be some online services, I'm just unaware of who they are or what the quality of their work is.
I myself am trying to find a good online scanning service for scanning large volumes of negatives and/or prints at a reasonable price. For me, price is much more important than quality (I just want to digitally preserve some family photos), since I have so much to scan. I'm really not sure where to go for this type of service, so if anybody knows of anything, please let me know (: