Very helpful:
Ars Electronica -- History.
Art and electronics.
Art and electronics -- Exhibitions.
Art and technology.
Art and technology -- Exhibitions.
Interactive art.
Interactive art -- Exhibitions.  
Multimedia (Art)
Multimedia (Art) -- Exhibitions.

Somewhat helpful:
Art and society.
Art and society -- Exhibitions.
Art and telecommunication.
Art and telecommunication -- Exhibitions.
Art -- Computer network resources.
Art, Modern -- 21st century.
Computer art.
Computer art -- Exhibitions.

Helpful for situating net art in the larger artistic context:
Art, Modern -- 20th century.
Art -- Philosophy.
Computer hackers.
Computer programming -- Moral and ethical aspects.
Conceptual art.
Conceptual art -- Exhibitions.
Open source software.
Performance art.
Video art.
Video art -- Exhibitions.

Unhelpful and even misleading:
Art and the Internet. You would expect this to be a goldmine, but it leads to only one title in the UNC system. Evidently this is a "rogue" LC subject classification.
Digital art. "Digital art" is a highly misleading term that will have you going in circles; 99% of the time it strictly means Photoshop and CGI, and not net art at all.
Digital art -- Exhibitions. Ditto.
Image processing. This also refers almost exclusively to computer graphics.
Image processing -- Digital techniques. Ditto.
Internet. A hopelessly broad term that returns too many results to process.
Interactive multimedia. Usually refers to CD-ROMs and Web design, not art.