You're probably going to have some trouble finding good games new or sealed unless you want to pay big bucks on ebay for them. Your best bet to find games at a good price though would be the flea market.
Half.com is a good place to find games complete with boxes/manuals. The prices won't be uber-cheap unless you're lucky but they're almost always better than e-bay prices for complete games, and most retails places only have cart only.
There is a mint (from what I can tell - it's behind the glass) copy of Paper Mario 2 at my local GameStop for ~$26...I'd recommend picking this game up if you can find a copy.