Replacing stem on non-oversize bars
25.4 ( or one inch ) is almost always an MTB handlebar diameter or a very old cheap bike. From 2003 and of good quality, it is almost certainly a 26mm handlebar. 

There are parts of the bike you can bodge without much concern and parts you can't. Anything in the fork/headset/handlebar area is not something you should try and "make work". If it goes wrong, you'll crash hard. 

You should still be able to find 26mm stems if you look hard enough. Here is a good source. 

http://store.velo-orange.com/index.php/components/stems/threadless-stems.html

They also sell the appropriate shims to use your bar with a 31.8 stem, but if it were my bike, I'd just pony up for a new 31.8 handlebar.