You'd be better off looking for UT2004. All the maps from UT2003, better gameplay, and since it was far more popular on the Mac, probably a lot easier to find used somewhere. As Altemose said, eBay and the Amazon Marketplace are your best bets.
I already have UT99, and UT2004, Both Mac and PC, And I Fixed my problem, I have UT2003 for PC, and followed a guide for converting UT2004 mac Demo to full version:
1. Install PC Version
2. Update PC Version to version 2225.0
3. Copy PC Version Files to Mac Version
4. Change the Version in info.plist to 2225.0
5. Run the 2225.3 patch for Mac
6. Put your CDKEY in file called cdkey in Sysytem
7. Have Fun!
(Make Sure you rename the Alias in the MacOS folder to Unreal Tournament 2003)
EDIT: The reason why I wanted UT2003, is really just for Kicks, I REALLY like making fun of the HUGE weapons and powerups (The Shock Ammo is almost as tall as the player) , and most of all, the announcer.