Lightology

A major source for lighting is located in Chicago: Lightology - an amazing three story showroom on Chicago Avenue.

Lightology is the largest importer and distributor of contemporary lighting in the US. It's glass walled atrium space encloses 15,000 square feet of showroom and a history of light exhibit and is topped with roof gardens.
Around since 1983 (it was formerly known as Tech Lighting), the current space opened in 2001.

Sales are divided between regular retail and architect/designers, and products include everything from chandeliers to desk lamps to transformers and dimmers.

The list of suppliers goes on and on - if you've been wanting to see a certain fixture in person, this is a good bet.

For those who can't make the trip to the showroom, the online store is extensive as well. Organized by type (recessed, table and desk, picture lighting, ceiling, wall, bathroom and mirror) it is searchable in many ways - by manufacturer, price range, color, even in-stock minimums.