Webscriptions has quite a few of Ms. Norton's older books available. (33 stories in 15 ebooks)
Crosstime - The Crossroads of Time and Quest Crosstime
Dark Companion - previously appeared separately as Dark Piper and Dread Companion.
Darkness and Dawn - previously published in parts as Star Man's Son and No Night Without Stars
From the Sea to the Stars - Sea Seige and Star Gate
Gods and Androids - previously appeared separately as Android at Arms and Wraiths of Time.
Janus - Judgment on Janus and Victory on Janus
Masks of the Outcasts - previously appeared separately as Catseye and Night of Masks
Moonsinger - Moon of Three Rings and Exiles of the Stars
Search for the Star Stones - originally published in parts as The Zero Stone and its sequel Uncharted Stars
Star Flight - previously published in parts as The Stars are Ours and Star Born.
Star Soldiers (Webscriptions Free Library)
The Game of Stars and Comets - Includes The X Factor, Voorloper, The Eye of the Monster, and The Sioux Spaceman
Time Traders (Webscriptions Free Library)
Time Traders II: The Defiant Agents and Key Out of Time
Warlock - previously published in parts as Storm Over Warlock, Ordeal in Otherwhere and Forerunner Foray
The price ranges from $4 to $6 each, some of her best works IMO and at these prices they rival the old Ace Doubles in value.
As with all Webscriptions ebooks they are available in many different formats (including Kindle) and always DRM free.
You will probably need to get these before Jan. 31, 2011 BTW