In person, I buy at B&H on W. 34th St. or Adorama on W. 18th here in New York City. Both are reputable and well stocked.
By mail, I frequent Samys Camera in CA, Unique Photo in Florham Park, NJ, and buy.com.
Bought all sorts of digital at all those places, from bodies to digicams, flashes to compact flash.
daloosh