You can buy 0.3mm stainless steel sheets for shimming, but as bullhorns are quite heavily loaded (your weight will create more torsion at the clamp than on an equivalent drop bar), I'd return or re-sell the bar, or replace the stem.
You can buy 0.3mm stainless steel sheets for shimming, but as bullhorns are quite heavily loaded (your weight will create more torsion at the clamp than on an equivalent drop bar), I'd return or re-sell the bar, or replace the stem.