state by state list of dealers, auctions and liquidations
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Omaha, Lincoln

Omaha:

Lozier (website)
(new manufacturer)

Superior Store Supply (website)
(new only)
Honorable mention: Lozier (website) is headquartered in Omaha, NE.

Lozier is a major manufacturer of gondola shelving, and their used gondola is the most prominent in the market.

It's great shelving, but being near Lozier's headquarters shouldn't help you when searching for used, nor does Nebraska have a prominent used fixture dealer.

Look for store fixture liquidations in Omaha, Lincoln, North Platte,  Grand Island and the Nebraska Panhandle.

Branch your scan out to nearby cities like Denver and Kansas City.
Try these "nearby" states:

Texas

Wisconsin

Kentucky