360 games do not work on the older Xbox. Micro$oft stopped publishing new games for the Xbox almost as soon as the 360 came out. Makes Bill Gates more money as people leave the older machine and buy new hardware to get the latest games.
I'd look in your local Game shop for older Xbox software as they do play n trade games on most formats ( they still had PS1 games late last year) or other places such a Gamestop, Gamestation, Play.com, Amazon, Loot etc. as well as Ebay and other online auction sites. You could also try Cash Convertors and Cash Generator but some of their stock can be very shoddy looking and uncared for.
I prefer to see what I'm buying esp. if it for a gift so I stick to high street shops. But I have bought second hand from Play trade and the games have been as good as new.