It is still very possible, with a little digging, to find some classic SSI games. I make no guarantees about any of these businesses, but here is information I have found. AKAIK, all software is new. Even if it isn't the version you're looking for, it will get you the manual(s), extras (maps, etc.), and, of course, those cool boxes!
[Page updated May 1, 2005.]
Atari 8-bit
Lists the following SSI titles:
Battle of Antietam, $14.95
Battle of Chickamauga, $14.95 (site lists this as an SSI game, believe it's actually by GDW or Rebel Charge has been mistyped!)
Computer Quarterback: 1984 Teams Disk, $4.95
Computer Quarterback: 1985 Teams Disk, $4.95
Cytron Masters, $9.95
Field of Fire, $9.95
Gettysburg: The Turning Point, $14.95
Kampfgruppe, $9.95
Mech Brigade, $14.95
Panzer Grenadier, $19.95
Rebel Charge at Chickamauga, $14.95
Shiloh, $9.95
War in Russia, $19.95
Warship, $9.95
Lists the following SSI titles:
Battle of Antietam, $19.95 [new listing]
Computer Baseball, 1983 Team Disk, $9.95
Computer Baseball, 1984 Team Disk, $9.95
Field of Fire, $19.95
Gettysburg: The Turning Point, $29.95
Kampfgruppe, $19.95
Kampfgruppe, Scenario Disk, $9.95
Mech Brigade, $19.95 [new listing]
Nam, $14.95
Panzer Grenadier, $14.95
Rebel Charge at Chickamauga, $19.95
Shiloh: Grant's Trial in the West, $19.95
Warship, $19.95 [new listing]
Atari ST
Lists the following SSI titles:
Curse of the Azure Bonds, $53.95
Demon's Winter, $9.95
Dragons of Flame, $9.95
Heroes of the Lance, $19.95
Hillsfar, $19.95
Phantasie, $29.95
Phantasie II, $29.95
Phantasie III, $29.95
President Elect, $19.95
Questron II, $19.95
Red Lightning, $19.95
Roadwar 2000, $19.95
Roadwar Europa, $19.95
Stellar Crusade, $9.95
Warship, $9.95
Wizard's Crown, $9.95
Video 61
Lists the following SSI title:
Waterloo, $19.95 [new listing, unsure if SSI or UK version]
Commodore 64
Lists the following SSI title:
Wargame Construction Set
Champions of Krynn clue book, $19.00
Lists the following SSI titles: [not sure if these are new or not, the "yes" indicates that a manual is included]
A D & D Champions of Krynn Disk Yes 14.95
A D & D Curse of the Azure Bonds Disk Yes 14.95
A D & D Death Knights of Krynn Disk Yes 14.95
A D & D Gateway to the Savage Frontier Disk Yes 14.95
A D & D Heroes of the Lance Disk Yes 14.95
A D & D Pool of Radiance Disk Yes 14.95
A D & D War of the Lance Disk Yes 14.95
Buck Rogers: Countdown to Doomsday, $9.95
Demon's Winter, $8.00
DragonStrike, $10.00
The Eternal Dagger, $8.00
Heroes of the Lance, $8.00
Panzer Strike!, $8.00
Questron II, $9.95
Roadwar 2000, $7.95
Wargame Construction Set, $9.95
Wizard's Crown, $7.95