NEONIXIE-L
Yahoo Group

Your FIRST stop should be to join this group!
They have large download and link sections.

Nixie Tube Cross-Reference Data

"A.J." Franzman's awsome table of nixie tube data.

Tubedata.org

Frank Philipse's massive collection of tube data.

Sphere

Very popular site. Data and tubes for sale.

Cathode Corner

David Forbes's collection of catalogs and datasheets.

Tube-Tester

Dieter's tubes for sale and lots of nixie data.