I have used B&H Photo for developing my slides for about 8-10 years now. I have a digital camera but I still use some slide film and this is the best service, IMO
Click on Slide and then the brand you want. I use Kodak or Fuji. You'll receive little envelopes from B&H that you'll put your roll of film in and mail to the processing lab. The Kodak goes to New Jersey or Maryland and the Fuji goes to Arizona. You'll get your slides back in the mail. It will probably take about 10 days to get your slides back, but the ones from New Jersey or Maryland will come back a little faster.