Look around at yard sales and used game stores. NES and SNES loose carts go for around $1-42 at yard sales around my area. Also, used game stores seem to undervalue oldies like VCS and Intellivision games, pricing most of the common titles at $1-$3 a piece.
Goodwill and pawn shops too. Lots of pawn shops will sell Genesis and PSX games at $1-$4 each.