How old is "old"? Sometimes gogamer will list some oldies in their 48-hour sales. I suppose they sell these same oldies all year 'round, depending on stock and supplier shipments.
Within the last several years, I bought several oldies from them:
Fallout 1 & 2 dual-jewel-pack <$5 (I had played #2, but not #1)
currently the Fallout Collection (1 & 2 & Tactics) is available at $20 everyday price
Planescape: Torment & Soulbringer dual-jewel-pack <$5 (for sale elsewhere now)
Arcanum (import version) was on sale last week (I already own it)
and is still available at $18 everyday price
others:
X3: Reunion
The Bard's Tale
Fable: The Lost Chapters
So you CAN still buy some of the classics (or at least some of the oldies that didn't suck) as new. Keeping looking for jewel case and "collection" re-issued distributions. The manuals will be on the CD/DVD, but most of the current manuals only add backstory and "flavor" to the game plot experience, so a pdf manual is acceptable.