Bicycle Mechanics - Stem shims?

I got some bullhorns for my bike, and they're supposed to be the proper size for my stem, but they're slightly too small around and shift if i apply a bit of weight to them after everything's tightened down. Back when I rode BMX we used to use Coke cans to shim things, but it's not seeming to do the trick with this. When I shim it to what seems to be the right diameter, the metal keeps bending and contorting before I can even get the bars to where they're supposed to be inside the stem. Does anyone know of anything else I can use as a make-shift shim that might be a bit more solid and able to be shoved in my stem without bending?

are you trying to use 25.4mm bars in a 26.0 stem? (or a 26.2 cinelli stem?)

you can use cut up cans, you just need more than what you're using now, or you can get a proper shim like this. (http://www.benscycle.net/index.php?main_page=product_info&products_id=353)

Unless they sent me the wrong size, they're definitely 26.0mm bars. That was the only size they came in too, so I'm pretty positive they're the right size. I've put them in two different stems just to see if it was the stem, and it was the same story with both.

Are they stamped with the size? Tried measuring them?

Neither are stamped with a size. I could try measuring them, but I'm just looking for a cheap DIY way to shim it all and keep my bars from shifting.