You can use a shim to allow a 31.8mm clamp diameter stem to work with 26mm clamp diameter bars. However, in my opinion you'd be better off finding a proper 26mm stem. What website were you looking at? There are some nice 26mm stems here: http://www.universalcycles.com/shopp...p?category=630

As you've found though, 31.8mm is the new standard so all of the newest and more expensive stems will be that diameter. Now might be a good time to swap handlebars too if you'd really like a 31.8mm stem.

If you decided to go with a shim anyway, here's what you need: http://www.universalcycles.com/shopp...0&category=784