Haawk58,
Go visit your local Train Club. I belong to the Black Diamond Society of Model Engineers in Bethlehem Pa. We have a company store that sells used model trains and stuff that were either bought as collections or donated to the club. HO cars usually were no higher than $2 or $3, O gauge were anywhere from $5 to $10. Many with no boxes were lower priced. In fact we used to give away a free car to kids when they came into the club. I have not been at the club for a few years after moving to Maryland (still a member though) but am a member of Delaware Seaside RR club and we do the same thing. If you are buying a load of cars make a offer too.......I bet the club takes it.
Forget retail purchases train clubs are the source.
JohnB